As of 0201 GMT, the main TAIEX index was down 7.45 points at 9238.75, after rising 0.19 percent in the previous session.
Subindexes showing major declines included the rubber index , which dropped 1.74 percent, and autos, which fell 0.92 percent.
Electronics and banks were flat.
Among individual names, smartphone maker HTC Corp registered a 1.85 percent fall, while the world's largest contract electronics manufacturer Hon Hai Precision Industry Co Ltd, which hosts its annual general meeting for investors on Wednesday, dropped 0.51 percent.
US stocks suffered falls on Tuesday after investors brushed off positive economic data and turned their attention to continued tensions in Iraq and the effect on global oil prices.
The Taiwan dollar was up T$0.041 to stand at T$29.990 to the US dollar.