A stress measurement method is provided of the present disclosure. The stress measurement method includes an image capturing procedure, a phase shift calculation procedure, an isochromatic intensifying procedure and a transformation procedure. The image capturing procedure is used to capture four light intensity images with four different phase angles of a sample. The phase shift calculation procedure is used to obtain an isochromatic retardation of the sample when the four light intensity images have sufficient light intensity values. The isochromatic intensifying procedure is used to calculate two enhanced light intensity values, the background of intensified isochromatic light intensity value and the amplitude of intensified isochromatic light intensity value to obtain an isochromatic retardation when the sample is in a low stress condition. The transformation procedure is used to transform the isochromatic retardation to a stress value of the sample.