The anticipated announcement comes amid nervousness about Europe's sovereign debt crisis and concerns over a possible Greek default.
Tokyo finished 0.23 percent, or 19.92 points, higher at 8,741.16, while Seoul was 0.89 percent, or 16.31 points, higher at 1,854.28 and Sydney gained 0.78 percent, or 31.6 points, to 4,071.8. Shanghai jumped 2.66 percent, or 65.20 points, to 2,512.96.
However, Hong Kong fell 1.00 percent, or 190.63 points, to 18,824.17, its lowest since July 2009.
"The market is expecting some stimulus plan from (Federal Reserve chairman Ben) Bernanke, so if that doesn't happen it will be an ugly night," said RBS Morgans principal investment adviser Chris Macdonald in Sydney.
Expectations have been high that the Fed will announce fresh stimulus measures after Bernanke left the door open for such a move at a speech last month.
"Given the weakness of the (recent US) economic data, the (Federal Reserve) is now almost certain to do something, but exactly what is still not entirely clear," said Paul Ashworth, an economist at Capital Economics, in a note.
A proposal to swap its short-term bond holdings to long-term -- dubbed Operation Twist -- appears to be the front runner instead of a third round of liquidity pumping, he told Dow Jones Newswires.
The thinking goes that lower longer-term interest rates will further encourage bank lending to companies and corporate investment.
The dollar fell against the yen ahead of the Fed announcement.
In early European trade, it bought 76.37 yen versus 76.46 yen late Tuesday in New York, while it was slightly down against the euro, at $1.3674 from $1.3703.
The euro remained under pressure, sitting at 104.43 yen against 104.76 yen.
Markets remain under pressure because of the ongoing eurozone crisis, with Greece the main focus of concern as it struggles to convince lenders it is able to put in place policies to meet terms of a huge bailout.
The country's Finance Minister Evangelos Venizelos is trying to unlock the latest eight-billion-euro ($11 billion) tranche of a rescue package that will help it avoid a default next month.
Most analysts fear that if Athens does default, the crisis could move to other economies, with Italy and Spain tipped to be next in line.
In Tokyo Toyota ended down 0.48 percent at 2,672 yen after the auto giant said it was temporarily shutting 11 of its 15 Japanese plants, which lie in the path of a powerful typhoon that made landfall in the afternoon.
Oil was mixed. New York's main oil contract, WTI light sweet crude for October delivery, was down 15 cents at $86.77 a barrel in afternoon Asian trade, but Brent North Sea crude for November turned higher by 36 cents to $110.90.
Gold fetched $1,805.80 an ounce by 0900 GMT, up from the $1,791.95 it was at by 1000 GMT Tuesday.
In other markets:
-- Singapore closed 0.39 percent, or 10.95 points, higher at 2,791.79.
Media giant Singapore Press Holdings rose 1.36 percent to Sg$3.74 and oil rig maker Keppel Corp dipped 1.04 percent to Sg$8.58.
-- Taipei gained 0.57 percent, or 43.03 points, to 7,535.88.
Taiwan Semiconductor Manufacturing Co rose 2.13 percent to Tw$72.0 while Quanta fell 2.63 percent to Tw$55.5.
-- Manila closed 0.37 percent, or 15.53 points, lower at 4,204.29.
SM Investments fell 2.1 percent to 520 pesos, geothermal power producer Energy Development was down 0.7 percent at 6.02 pesos and Alliance Global dropped 0.9 percent to 10.06 pesos.
-- Wellington rose 0.55 percent, or 18.21 points, to 3,308.83.
New Zealand Oil and Gas fell 4.3 percent to NZ$0.67 while Telecom added 1.2 percent to NZ$2.60 and outdoor apparel retailer Kathmandu rose 0.5 percent to NZ$2.20.
-- Kuala Lumpur closed up 0.59 percent, or 8.40 points, at 1,419.04.
Plantation firm IOI Corp. gained 0.6 percent to 4.71 ringgit, while utility company Tenaga Nasional inched up 0.4 percent to 5.12 ringgit. Shipping company MISC lost 1.7 percent to 6.76 ringgit.
-- Jakarta fell 1.46 percent, or 54.62 points, to 3,697.49.
Bank Mandiri shed 1.6 percent to 6,200 rupiah and car distributor Astra International dropped 2.1 percent to 64,050 rupiah.
-- Bangkok rose 0.32 percent, or 3.31 points, to 1,029.59.
