Abstract: This paper mainly deals with how to determine paleosalinity by means of δ13C. In addition, the determination of paleotemperature is also dealt with by the use of δ12O. By a thorough analysis of the systematic data,the author has reached the following conclusions; ( 1 ) Both δ18O and δ13C values relate to the salinity of medium and the chang shows a trend that the higher the salinity, the higher the δ values. Therefore, it is feasible to distinguish marine limestone from freshwater limestone of Jurassic period of more young ages by the formula of Keith and Weber ; Z = 2.048 (δ13CpBD + 50) + 0498 (δ18OpBD + 50 ) .This paper gives an example of analysing the beachrocks in Hainan Island, South China.It shows the relationship between isotopes and salinity. ( 2 ) The δ18O and δ13C values of the tediments ( rocks ) older than Jurassic have been greatly changed because of the displacement of the isotope elements during the long geological time, so that the paleosalinity determined by theδ13C and δ18O values of ancient rocks would not be reliable. Through the study of this aspect the author suggests that in the process of dia-genesis,the exchange of carbon isotope between rock and medium is much weaker than that of oxygen isotope. Since the Cambrian period the δ13C value has not notably changed,hence, it is feasible to determine paleosalinity by using the δ13C values. (3 ) The exchange between O18 and O18 after sedimentation is strong and the δ18O value changes with the geological ages, that is , the δ18O value increases from old to new rocks, so that the δ18O value of ancient marine limestone would approximately be equal to that of freshwater limestone. Therefore, the δ18O values in an- cient rock can not be used on determining paleosalinity. ( 4 ) As the δ18O value increases with geological ages from old to new, so that it could not be used to determine paleosalinity.Since the δ18O value of ancient fa-cies rocks relates to the diagenetic environment so it can, no doubt, reflect the strength of diagenesis