Abstract: Described in this paper theoretical limit on resolution of optical microscope , transmission microscope and scanning microscope. Discussed conditions optimum resolution, that obtained from DX-5 scanning electron microscope. The optimum resolution is DX-5 scanning electron microscope is 60? (see Fig. 1), its magnification is 10×~100000× （see Fig.3.4）. The microscope of clay mineral, marine soils from the south China Sea, and gysum-diatomite have been studied with DX-5 scanning electron microscope. The results obtained from our research show that DX-5 scanning electron microscope is valid for the research of the microstrurect of soils in rock and soil mechanics.