Abstract: There have been many papers reporting experimental of transition in laminar-turbulent boundary layer. Recently, great progresses have been obtained on the investigation of transition route by means of surface hot films. In order to study the effects of various parameters, such as Reynolds number, pressure gradient, noise, temparature etc., on the transition and to monitor transition automatically in wind tunnel testing and flying tests, it is necessary to give a reasonable definition of the transition" point" and to find out a corresponding measuring technique. This paper introduces a simple and reasonablly accurate measuring technique, i. e., the probability density technique, to determine the location of Klebanoff transition.