Abstract: The mineral compositions of 34 gneiss samples are tested through thin section identification of polarizing microscope and X-ray powder diffraction phase analysis. Comparison shows that 18 of the 34 rock samples were named consistently by the two methods, while the other 16 were differently. The advantage of polarizing microscope identification is that it can accurately determine the structure and texture of rocks, identify most of metallic minerals and tourmaline, sphene, apatite and epidote as well, and effectively distinguish muscovite from biotite. The X-ray powder diffraction can exactly distinguish stratiform silicate minerals such as chlorite, montmorillonite and mica, and determine the mineral types and contents of potassium feldspar, albite and quartz. The results suggest that gneiss identification should combine the two methods to better determine mineral compositions and provide more objective data and analysis conclusions for geological study.