At 0324 GMT, the benchmark SET index was down 0.8 percent at 1,081.85, reversing a modest gain at the opening.
Bank shares dropped 1.4 percent and energy shares fell 0.9 percent.
Bank and energy shares led the market rally on Monday, when investors cheered the resounding victory by the opposition Puea Thai Party in the weekend general election because it removed some political uncertainty after years of instability.
Shares in developer SC Asset Corp dropped 0.7 percent to 15.3 baht after a 12.4 percent surge on Monday. The new government will be led by Yingluck Shinawatra, a former top executive of the company and sister of ex-premier Thaksin Shinawatra.
Brokers said market attention now would be on the appointment of key economic ministers.
Copyright Reuters, 2011