The soot formation and evolution characteristics in premixed methane/ethylene-oxygen-argon flames are studied experimentally and numerically. The soot particles sampled with an in-situ probe sampling method are measured for different light hydrocarbon fuels (i.e., methane and ethylene), heights above the exit surface of the burner (HAB), equivalence ratios, phi and Reynolds numbers of flame jet, Re-j A novel stochastically weighted operator splitting Monte Carlo (SWOSMC) method coupled with detailed soot model is developed to simulate the evolution of soot particle size distribution (PSD) in premixed methane/ethylene-oxygen-argon flames. The flame temperature decreases while geometric mean diameter of soot particles increases with increasing phi. With increasing phi condensation and nucleation processes are enhanced in methane flame while coagulation and nucleation processes are enhanced in ethylene flame. Hence, these processes lead to an increase of total soot number concentration for methane flame but a decrease for ethylene flame. A distinctly different variation trend of the normalized total number density of soot particles for methane flame along with HAB can be found for low and high studied Re-j ranges. Similarly, a distinctly different variation trend of geometric mean diameter of soot particles for ethylene flames along with HAB can also be found for low and high studied Re-j ranges. The numerical simulation results obtained via the fully validated SWOSMC method show excellent agreement with experimental results in the present study. Consistent with the experimental results, the numerical simulation results show that both condensation and nucleation rates increase and become dominant processes in the methane flame while coagulation rate increases in ethylene flame with increasing phi Meanwhile, the simulated nucleation and coagulation rates along the centerline of both methane and ethylene burner flames are reversed with increasing Re-j The present study not only verifies the competition phenomenon among different soot dynamic processes but also shows that this competition can be reversed for different Re-j ranges. These results show that the soot formation and evolution characteristics in the studied premixed methane/ethylene-oxygen-argon flames are determined by competition among different soot dynamic processes.