A measurement for the time resolution limit produced by a photomultiplier, with the accuracy of ± 2～5×10-9 Sec., has been made. With the aid of a hydrogen lamp, illuminating at 5～8×10-8 Sec., the flight time of photoelectrons of serveral kinds of photomultiplier has been determined by employing the oscilloscopic method; an investigation as to the effect of the outer circuit of RCA-5819 for resolution time has been made and the better working state suitable for instant observation for the spectrum excitation process has also been determined.