The coalescence process of poly (ethylene-co-vinyl acetate) (EVA) and poly (ethylene-co-octene) (EO) dispersion particles was monitored in situ using tapping-mode atomic force microscopy (TMAFM) equipped with a miniature hot stage. This work describes the effect of particle size on the film formation temperature based on direct experimental observation, clarifying further the debate about particle size effect on minimum film formation temperature (MFFT). The results suggest that semicrystalline polyolefin particles have similar deformation temperature dependence. Smaller particles tend to deform faster than larger particles, which is attributed to their smaller mass. Furthermore, morphology changes and mechanical property development associated with the film formation process are also discussed. The TMAFM technique is shown to be very useful in gaining insight into the film formation mechanism, which will provide guidance in future practical applications with polyolefin dispersions.