The initiation, propagation and broadening of crazes in polycarbonate were studied with the aid of polarizing microscopy. A loading instrument supplying uniaxial stretch is designed to match polarizing microscopy. With polarizing microscopy, online observations of crazes and stress field can be done simultaneously. Crazes appear in bilateral anteriority of the crack tip and direction of length is perpendicular to the maximum principal stress. Stress on the boarder of the crazing zone is taken as the initiating point of the crazes and length to time of crazes follows exponent law. The initiation, propagation and broadening of crazes is a time-dependent process.