The index fell after rising by 1.77 points or 0.11 per cent on Tuesday.
In the morning session, Krungsri Securities forecast the SET Index on Wednesday would fluctuate between 1,625 and 1,645 points.
It said the index gained positive sentiment from rising oil price in line with economic recovery and mass buy-ups of stocks whose third-quarter profit is expected to grow.
"However, investors' mass sell-offs of stocks to escape risk before the meeting of the European Central Bank on October 28 and Federal Reserve on November 2-3 would pressure the index," Krungsri Securities said.
The 10 stocks with the highest trade value today were BANPU, KCE, SCGP, KBANK, AOT, IVL, SCB, PTT, PTTEP and DELTA.
Other Asian indices were down with one exception:
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