Based on the knowledge of chemical composition and X ray analysis of <2 μm clays in agric horizon of the black soil in Northeast China, clay mineral composition and its spatial variation of 15 black soil samples collected from different latitudes was studied, and its affecting factors and evolution mechanisms explored. Results reveal that the clay minerals in agric horizon of the black soils in Northeast China are of the type of illite-Smectite/Illite mixed layer mineral, but their composition varies from region to region. At high latitude they are of the type of illite-Smectite/Illite mixed layer mineral, and at low latitude of the type of S/I mixture-illite-vermiculite. Factors causing spatial variation of their composition are mainly precipitation, temperature, and type of crops. As they vary sharply from region to region, clay minerals in the black soil of Northeast China evolve from illite toward S/I mixture or vermiculite, resulting in significant regional variation of clay mineral composition with latitude.