Photoconductive properties of diamond optical switch made by chemical vapor deposition method were investigated. A new configuration of the diamond gap was proposed to reduce the surface leakage current and avoid surface flashover. This technology made it possible to apply static high electric field up to 2×106V/cm. The dependence of the mobility‐lifetime product (&mgr;&tgr;) on the grain size was measured for a wide range of electric field. The &mgr;&tgr; value was increased to be linearly proportional to the electric field for every grain size sample, and no saturation was measured even at a high electric field ofE=3×105V/cm. Larger grain size samples had larger &mgr;&tgr; values. The grain size dependence was attributed to the decreasing of the mobility or the lifetime inside the grain not due to the increasing recombination ratio at the grain boundary in smaller grain size samples. ©1995 American Institute of Physics.