Poly-P-phènylene sulphide (PPS) is a highly crystalline resin. The crystallinity directly affects its physical and mechanical properties and also its processing. However, in literatare, only a qualitative description or a semi-quantitative orystallinity index value had been found. The present paper reported the continuous and cumulative, counting method, first and successfully, to determine the orystallinity of PPS, by X-Ray diffraction. According to this method, virgin powder of PPS possesses a orystallinity noar 75%.After heating 6 hours in air, at 360℃, PPS is transformed in to an amorphous solid by oxidative and thermal orosslinldng. The intermediate orystallinity had been found between the two extremes,by different heating conditions.