The response characteristic of a LTGaAs photoconductive switch formed in a cop lanar waveguide at different voltage bias is studied with photocurrent autocorre lation measurement technique. The experimental results show that the switching t ime increases from ～200fs to ～750fs, when the bias voltage ascends from 0.5×104 to 9.5×104 V/cm. This phenomenon is attributed to the increase of carr ier capture time arising from the potential barrier lowering (FrenkelPoole eff ect) and the fieldenhanced thermal ionization.