Surface seals and crusts, formed as a result of aggregate disintegration and compaction during a rain or irrigation event, are higher in shear strengths, greater in bulk density, finer in pore and lower in infiltration rate than the underlying soil. Soil surface strength and micromorphological photos are important indices and direct representations in the research on soil crust. Two soils, Lu Soil and Black Soil, typical of arable soils in China, were used to investigate effects of wetting rates (WR) of (2 (low) or 50 (high) mm h^-1) and cumulative rainfall on crust formation, under simulated rainfall with intensity of 60 mm h^-1. The experiment indicated that the effect of WR was negligible, but that of cumulative rainfall was significant on crust formation of Lu Soil. The crust formation processes of Black Soil were affected mostly by WRs. High WR had much greater effect than the low WR. Disintegration of soil aggregates plus rainfall of high intensity led to formation of crusts of high surface strength. Thus, the relative importance of WR and rain depth depends on stability of soil aggregates. For soils with stable aggregates, the impact of raindrops impact alone is not sufficient to destroy aggregates and form crusts. Fast wetting of dry soil is essential for aggregate breakdown and crust formation. For soils with unstable aggregates, the impact of raindrops is sufficient for seal formation.