A new in situ method for particle sizing is described. The aerosol particles are heated up by a laser pulse. Their thermal radiation is detected by a photomultiplier. Numerical analysis of the measured signal based on a solution of the first kind Fredholm integral equation results in the particle size distribution. The method is characterized by measurement time of less than some microseconds and applicable for measuring particle size distribution in various Size ranges down to some nanometers. An example of the size distributions of soot particles measured in a sooting propane diffusion flame is illustrated. Other possible applications of the method are discussed.