The Mn-Ga alloy displays high coercivity and a relatively large magnetic anisotropy. These characteristics suggest that it is a good potential magnetic material for the future. Herein, we prepared Mn-20at%Ga magnetic nanocomposites through mechanical alloying using high-energy ball milling. After ball milling for a total time of 3.5 h, the powder was placed in a cylindrical die with an inner diameter of 10 mm, and was pressed into a bulk specimen of size ? 10×30 mm. The samples were sintered in the temperature range from 300℃ to 415℃, and annealed from 2h to 8h. This paper focused on the formation of magnetic phases and their sizes, magnetic properties under various heat-treatment conditions. The results showed that the main magnetic phases in Mn-20at%Ga alloys were Mn3Ga and Mn0.85Ga0.15, in addition to the MnO2 phase caused by oxidation. Mn3Ga and Mn0.85Ga0.15 were generated respectively from the high Ga content region and low Ga content region caused by the high-energy ball milling. The high Ga content region appeared to be irregular particles, whereas the low Ga content region appeared to be located in the interparticle region. The magnetic properties were measured at room temperature using a vibrating sample magnetometer. To evaluate the magnetic properties quantitatively, the coercivity, remanence, and energy product were deduced from the data of hysteresis curves. The remanence, coercivity, and maximum energy product can be improved by enhancing the annealing temperature into a proper range. Enhancing the annealing time properly also benefited the improvement of remanence and maximum energy product. However, coercivity change due to the annealing time was negligible. The optimal magnetic properties in this research were obtained at an annealing temperature of 385℃ and annealing time of 6h, which showed a remanence of 63.21 emu/cm3, a coercivity of 8.1 kOe, and a maximum energy product of 0.15 MGOe. The size change of Mn3Ga nanophase due to annealing conditions was small. However, the size of Mn0.85Ga0.15 nanophase was decreased due to a proper enhancement of annealing temperature and annealing time, which corresponded to the increase of coercivity. The crystal size decrease of Mn0.85Ga0.15 benefited the enhancement of magnetic properties.