To improve the sinterability of powders fabricated by the conventional mixed-oxides method, ultrafine Mn-Zn ferrite powders were hydrothermally synthesized from metal nitrates solution using ammonia as a precipitant. The R value (alkalinity) was introduced to adjust the amount of added OH-in the reaction suspension. The characteristics of the powders synthesized at different hydrothermal conditions and the properties of the sintered bodies were investigated. The results show that the R value and hydrothermal time have a great effect on the compositions and phases of hydrothermally synthesized Mn-Zn ferrite powders. Powders synthesized from a starting suspension with a higher content of Zn ions (or lower content of Mn2+) may approach to a stable spinel structure with a lower Mn/Zn ratio as the hydrothermal time is longer. Factors affecting the position of the diffraction angle (2theta) of the spinel Mn-Zn ferrite (311) of powders may include both the compositions of spinel ferrite structure and crystallite sizes (or particle sizes) of powders. Some possible reasons were suggested to explain the dependence of composition and phase of hydrothermally synthesized Mn-Zn ferrite powders on the R value and hydrothermal time. The temperature that the green compact begins to shrink at increases with increasing R value, and ranges from 510degreesC (R = 2) to 650degreesC (R = 6). After being sintered at 950degreesC for 2 h in N-2 atmosphere, the relative sintered density of each specimen reaches a value of 94.5-99.8%.