Evolution of particle size distribution function (PSDF) was studied in premixed stretch-stabilized flat flames for the first time. The purpose is to demonstrate that stretch-stabilized flames can broaden the experimental flame condition space for studying soot formation in a pseudo-one dimensional flow configuration. PSDFs were measured in three series of atmospheric-pressure ethylene–oxygen–argon flames with maximum temperatures around 1980, 2000, and 2160 K. The measured PSDFs show a strong effect of flame temperature as nucleation and growth of soot is found to be suppressed towards high temperatures. Simulations using a population balance soot model show reasonably good agreement with the 1980 and 2000 K series of flames, but it significantly overpredicts the number density and size of soot in the highest temperature flame. Numerical tests suggest that the discrepancy can be caused by high-temperature reversibility in surface growth and other processes.