The incorporation of magnesium in the synthetic apatite has been associated with biomineralization process and osteoporosis therapy in human and animals. Magnesium easily replaces calcium in the apatite lattice and influences or controls the hydroxyapatite crystallization processes. In this work, Mg-substituted calcium deficient apatite, with Mg/Ca ratio = 0.1, 0.15 and 0.2 were synthesized by precipitation method. Then, sintered at 1000 oC and compared with a commercial product labeled as tricalcium phosphate sintered at the 1000 oC. The sintered products showed tricalcium phosphate (β-TCP) structure. The Mg2+ substitution in the Ca(4) and Ca(5) sites of β-TCP and the lattice parameter changes were estimated using the Rietveld method. Using this method, the formulas Ca2.73(Mg0.27)(PO4)2, Ca2.71(Mg0.29)(PO4)2 and Ca2.70(Mg0.23Mg0.07)(PO4)2 were calculated for the samples with Mg/Ca ratio = 0.1, 0.15 and 0.2 respectively.