Variation of forms, contents, and bioavailability of soil inorganic phosphorus in upland Calcic Kastanozems of the Loess Plateau under long-term fertilization were studied. The results showed that the fraction of Ca₁₀-P occupied the largest percentage, or 57.7 % of the total inorganic phosphorus in the soil. O-P came the next, accounting for about 17.9 %, and then was followed by Al-P, Fe-P, Ca₈-P and Ca₂-P, which accounted for 5.9 %, 5.7 % and 10.1 %, respectively. In the fertilization treatments, Treatment M+NP was the highest in inorganic phosphorus, whatever the forms; Treatment N the lowest in Ca₂-P, Ca₈-P and Al-P, and Treatment CK the lowest in Fe-P, O-P and Ca₁₀-P. Long-term fertilization also affected relative contents of inorganic phosphorus of different forms. The phosphorus consuming treatments (N & CK) resulted in decrease of mainly Ca₂-P, Ca₈-P and Ca₁₀-P, whereas the treatment of phosphorus application did mainly in degradation of Ca₁₀-P and accumulation of Ca₂-P. Compared with the findings in 1990, phosphorus of all fractions decreased in CK. In Treatments N, phosphorus of all fractions, except O-P and Ca₁₀-P, declined, but in Treatments NP, S+NP, M and M+NP, all fractions showed increasing trends. Different treatments had different effects on available P and potential phosphorus, which, however, had nothing to do with unavailable P. The correlation analyses of different forms of inorganic P with crop yield were conducted. The correlations of Ca₂-P, Ca₈-P and Al-P with crop yield reached a high level of significance. Among the three, Ca₈-P was the best correlated with yield, and Fe-P, O-P and Ca₁₀-P were also significantly correlated with yield.