A new method combining light transmission and bubble pressure has been tried to determine the pore size distribution of microporous membranes. The method is based on a simple phenomenon that the light transmitivity through an opaque porous membrane increases as its pores are filled with a transparent liquid. The pore size distributions of Durapore HVHP and Sartorius PTFE membranes were tested using the new method and the results were compared with those obtained by the fluid displacement and the mercury intrusion methods.