Calcium polyphosphate (CPP) may be a promising bone substitute with controllably degraded ability. In this investigation, the effects of sintering temperatures on CPP's phase transformations and microstructure parameters, such as the distribution of crystallite size and micro-strain, were investigated by X-ray diffraction (XRD). The qualitative phase analysis and quantitative phase analysis based on reference intensity ratio (RIR) method were conducted for the CPP sintered at 585, 600, 650, 700, 750, 800 and 900degreesC. The distribution of crystallite size and micro-strain were calculated with the Warren-Averbach Fourier Transfer method. The results demonstrated that the transformation of amorphous CPP to semi-crystalline CPP occurred below 585degreesC, and semi-crystalline CPP to gamma-CPP at temperature of 585-600degreesC; gamma-CPP to beta-CPP at 585-700degreesC. CPP sintered between 600-700degreesC were composed of both gamma-CPP and beta-CPP, and the mass fraction of beta-CPP increased with rising of temperature. Above 700degreesC, the sintered CPP only contained beta-CPP. At different ranges of the sintering temperature, the average crystallite size (D) and micro-strain (epsilon) showed significant difference, for example, D and epsilon is about 2.9 nm and 1.68% at 585degreesC, but D and epsilon was 8.0 to 8.7 nm and 0.159 to 0.134% at 600 and 700degreesC, respectively. The results of the phase transformations and the variations of microstructure parameters in the present study may be able to provide some fundamental data for explaining CPP degradation phenomena. (C) 2004 Kluwer Academic Publishers.