The results of two years field experiments with corn on cultiveted upland red earths in Zhejiang had showed that the extraction of available P with acidic extractants was better than that with alkaline extractants. In oder to farther prove this conclusion, four different experiments were conducted: (1) fractionation of soil P associated with statistical calculation, (2) determination of correlation coefficients between different forms of P and the results of different extraction methods, (3) calculation of the difference of Al-P, Fe-P and Ca-P of three soils before and after glowing a season of wheat, (4) isotopic dilution method of Dumbar and Baker. Results of the experiments showed that the principal form of available P in cultivated upland red earths is Al-P. Due to the acid soluble property of active solid Al-P, acid extractants are more satisfactory and reasonable. In this paper, the authors recommend Bray I and Mehlich Ⅲ as the most, convineant extractants for available P in cultivated upLlnd red earths in Zhejiang. Calibration scale of these two methods and critical values of P in soil for corn by five method are given in this paper.