Optical emission spectroscopy is an effective tool to measure plasma parameters and species distribution in plasma. Optical emission spectroscopy is used to measure the microwave plasma of CH4/H2 in situ. The internal radicals, the influence of methane volume fraction on radical emission intensity, and spatial distribution of the radicals and the influence of methane volume fraction on the spatial distribution in MPCVD plasma are investigated. The results show that the optical emission intensity of C2 in plasma increases with the methane volume fraction increasing obviously. Intensity ratios of CH, Hβ, Hγ to C2 reduce with increasing methane volume fraction. The spatial distributions of the radicals especially of C2 become inhomogeneous with the methane volume fraction increasing.