Sol-Gel chemistry has been used to prepare undoped and Mg-substituted biphasic calcium phosphates (BCP) ceramic composed of hydroxyapatite (HAp) and whitlockite (beta-TCP) phases. Different series of samples have been synthesized with different Mg doping level (from 0 to 5 atomic % of calcium atoms substituted) and different temperature of calcinations (from 500 to 1100°C). All the powdered samples were systematically treated by Rietveld refinement to extract the quantitative phase analysis, the structural and microstructural parameters, to locate the Mg crystallographic sites and to refine the composition of the Mg-substituted phases. The temperature dependence of the weight amount ratio between HAp and beta-TCP is not monotonic due to the formation of minor phases as Ca2P2O7, CaO, MgO, CaCO3 and certainly an amorphous phase. On the other hand the Mg stabilizing feature on the whitlockite phase has been evidenced and explained. The mechanism of stabilization by small Mg2+ is different to this by big Sr2+. Nevertheless, in both cases, the whitlockite stabilisation is realised by an improvement of the environment of the Ca4 site unusually face coordinated to a PO4 tetrahedron. The substitution of Mg atom in the Ca5 site allows to improve considerably the bond valence sum of the unusual Ca4 polyhedron. The temperature of calcinations combined with the amount of introduced Mg atoms allow to monitor the phases composition of the BCP ceramic as well as their microstructural properties. The bioactivity properties of the BCP samples are improved by the presence of Mg atoms in the structure of the beta-TCP phase. The mechanism of improvement is mainly attributed to an accelerated kinetic of precipitation of a calcium phosphate layer at the surface comprising HAp and/or beta-TCP phases.