Classical differential interference contrast (DIC) microscopy is an excellent tool for rendering high-contrast images to observe transparent specimen. Though the monochromatic shadow-cast image qualitatively reflects the specimen configuration, its phase can hardly be extracted, inevitably limiting its application in quantitative measurements. In order to extend its application, we propose a quantitative DIC (qDIC) microscopy technique, which can extract the specimen phase from phase-shifting beam-shearing interference images combining principal component analysis (PCA) and phase integration methods. Proved by both simulations and experiments, the PCA-based qDIC microscopy can rapidly and accurately retrieve the quantitative specimen phase. The new proposed technique shares with conventional DIC the advantage of high-contrast imaging, especially for transparent label-free specimens, but transforms DIC microscopy into a quantitative phase microscopy technique. We envision PCA-based qDIC microscopy as a future quantitative biological imaging modality.