The effect of 2% of $P_2O_5$ substituted in gypsum(5 mol% as $HPO_3$) on the early hydration characteristics of $C_3A$ were investigated and then gypsum only and gypsum mixed with $Ca(H_2PO_4)_2$.$H_2O$(2% as $P_2O_5$) were studied separately for comparison. The early hydration reaction of $C_3A$ with each gypsum were examined by measuring the rate of heat libera-tion of hydration with calorimeter and by analysing the hydration products with X-ray diffractometer and differential thermal analyser. It was shown that phosphogypsum substituted with $P_2O_6$ in the crystal lattice accelerated exceedingly the formation of the ettrignite and following hydrated products. However the other gypsum especially gypsum without any phosphate delayed the formation of various hydration products, It was consequently suggested that when phosphogypsum are used as the retarder of cement setting its reta-rding effect for setting of cement is not ascribed to the retardation of ettringite formation.