Mechanochemical synthesis of Mg-ferrite (MgFe2O4) and Ni-ferrite (NiFe2O4) was investigated and optimized. The starting reagent (oxides and hidroxicarbonate) were mechanical grinded in two mills (centrifugal and vibratory) with different conditions of mass-powder:balls ratios. Also, we have studied the influence of different balls diameters (from 2.5 to 20 mm), materials of balls and container (stainless steel and agate) and dry and wet milling conditions. By means of XRD, we examinate the evolution with the milling time of both starting crystalline reagent and the new emerging phases. In order to stablish the existence of a critical crystalline size for the ferrite formation, we used the Scherrer formula. We have observed very different behaviour between both systems. In the case of Mg-ferrite the most important peaks of the ferrite appeared after 25 hours in the case of centrifugal mill, while using vibratory mills we have not detected the presence of the ferrite after 160 hours. The formation of the Ni-ferrite occurs at lower times (3 hours) and the difference between the use of vibratory and centrifugal mill were negligible. The reaction was also followed by means of the evaluation of the hidroxicarbonate amount reacted during the milling. It was quantified by means of thermogravimetric analysis (TGA).