Abstract: A burner-stabilized stagnation flame probe system with scanning mobility particle sizer（SMPS） technique was used to investigate the evolution of soot size distribution functions for laminar premixed ethylene and ethane flames separately. To analyze the experimental observations, numerical simulations were carried out using the OPPDIF code and the high-temperature C₁-C₄ reaction model USC-mech II. By keeping the same C/O ratio and maximum flame temperature, the soot particle size distribution functions（PSDF） of the two flames evolve very similarly as a function of height above the burner surface except for an offset by 1~2mm, which was due to the slightly different residence/reaction time and temperatures at the same height, according to our analysis about the temperature and species profiles.