Abstract: A quantitative analysis of microstructure of weathered granite under optical microscope by using Qwins image processing system is done in the paper and quantitative indexes, such as content, grain size, orientrf porosity, shape factor etc. are listed. According to the analysis result, the granite in Kwun Tong is moderately weathered fine-grained granite, and the granite in Shek Kip Mei is highly weathered coarse-medium-grained granite.