The ultrafine activated rock phosphate (UARP) was prepared and its phosphorus availability and fertilizer efficiency as affected by amendment of activator and grinding was studied using chemical analysis, infrared spectrum (IR) analysis and pot experiment. Results show that the fractions of available phosphorus and labile phosphorus in UARPs increased by 45.12%~58.68% and 169.4%~203.6%, respectively. The fraction of water-soluble phosphorus in UARPs increased markedly with the amount of activator added. When the activator was added to make 5% of the UARP, the accumulative water-soluble phosphorus release rate of the fertilizer increased by more than 84 times. Proper extension of grinding time contributed to release of phosphorus from the product, but the effect was not significant. Infrared spectrum (IR) analysis shows that the PO₄^3- and HPO₄^2--related characteristic absorption peaks of UARPs were much higher in intensity than those of common RP, thus forming new characterastic absorption peaks, which indicates that phosphorus in rock phosphate powder is turned to be available. Results of the pot experiment indicate that treatment UARP was much higher than treatments RP and UFRP in corn biomass and fertilizer use efficiency, and even higher than treatments SP (superphosphate) and DAP (diammonium phosphate), and that UARP significantly increased available phosphorus by reducing fixation of phosphorus in the soil. The relatively high content of labile phosphorus and the sustained moderate phosphorus supply level of water-soluble phosphate in UARP were the main causes of its high bioavailability.