The spatial coherence properties of high power vertical cavity surface emitting lasers (VCSELs) are studied based on the theorem of partially coherent light proposed by Van Cittert-Zernike. The interference stripe patterns are gauged using the Young′s double-slit experiment for the devices of 980 nm wavelength VCSEL single emitters. Then interference patterns are switched to grayscale by intensity distribution patterns for data collection. In this paper, the integral average value method is proposed to calculate the degree of spatial coherence. The proposed integral average value method and traditional average method are used respectively to calculate the light intensity of the pattern. The results are compared with those calculated by the theoretical value of Van Cittert-Zernike theorem. The VCSEL optical apertures′ influence on coherent characteristics are also discussed. The experimental results show that, a relative error within 2.5%～9.4% is reached by our integral average value method. On the contrary, a relative error of 7.5%～67.4% are got from the traditional average method. Apparently, traditional average value method causes the error generally 1.5～27 times worse than our integral average value method. Moreover, for single emitter VCSELs with optical aperture from 200 μm to 500 μm, the smaller the optical aperture are, the larger the coherent degrees (between 0.731～0.426) are. The conclusion can be wildly used on theoretical and experimental design for VCSEL coherent arrays.