The optical, thermal and electrical properties of ultra-thin two-dimensional(2D) crystal materials are highly related to their thickness. Therefore, identifying the atomic planes of few-layer crystal materials rapidly is crucial to fundamental study. Here, a simple technique was demonstrated based on optical contrast for counting atomic planes(n) of few-layer MoSe_2 on SiO_2/Si substrates. It is found that the optical contrast of single-layer MoSe_2 depends on light wavelength and thickness of SiO_2 on Si substrate. The data calculated based on a Fresnel law-based model as well as atomic force microscopy(AFM) measurements fit well with the values measured by spectroscopic ellipsometer. Furthermore, the calculated and measured contrasts were integral and plotted, which can be used to determine the MoSe_2 atomic planes(1≤n≤4)accurately and rapidly.
