[en] Two-dimensional (2D) transition-metal dichalcogenide (TMD) exhibits thickness-dependent optical, electronic, mechanical, chemical and vibrational properties, and a fast, non-destructive thickness characterization technique is very important for fully understanding the thickness-dependent properties of TMD nanosheets. MoS_2 and WSe_2 nanosheets with different layer numbers are fabricated by a mechanical exfoliation method and transferred onto SiO_2 (300 nm)/Si substrate. In this work, it is found that the Raman area (i.e. the integrated intensity) ratio between the Si peak from SiO_2/Si substrate underneath TMD nanosheets (A_S_i) and that from bare SiO_2/Si substrate (A_S_i_(_0_)), A_S_i/A_S_i_(_0_), can be used to accurately identify the layer number of those TMD nanosheets. A_S_i/A_S_i_(_0_) is then calculated using the Fresnel equations, the absorption of TMD nanosheets is included in the calculation, which has not been considered in previous work. The consideration of absorption is especially necessary for TMD which has a quite large extinction coefficient. Identification of single-layer TMD with different complex refractive indices on SiO_2/Si substrate and on Si substrate using this method is given. The effects of thickness of SiO_2 and excitation wavelength on the layer number identification using this method are also discussed. (paper)