Wavy extinction in quartz is a good indicator for the magnitude of deformation. The intensity of wavy extinction (IWE) in quartz was quantitatively measured using polarized optical microscope, digital camera and NIH Image. Quartz-grain showing a maximum interference color is appropriate for the measurement of IWE. A traverse for the measurement is set across the parts showing distinctive wavy extinction, where subgrain boundaries in deformed quartz generally intersect with the line. The brightness of each part showing wavy extinction is measured at the extinction position of a subgrain. A gradation of brightness is treated as 256 gradient in computer. IWE is defined by the formula, IWE=tan (magnitude of brightness/1000*length of the line for measurement), where the length is scaled by millimeter. Based on this method, we can give an intensity of wavy extinction to deformed quartz grains, and it has been shown that the mylonitic and weakly mylonitized granitic body adjacent to the Tanakura shear zone in northeastern Japan can be divided into several zones of deformation intensity. This method can be applicable to many kinds of deformed minerals to describe the degree of deformation.