Mn3Ga films were grown by rf-sputtering on SiO2/Si(001) substrates at 300 °C. Ultra-thin films with 2 nm of thickness are composed of small grains with an in-plane diameter ~27 nm. The film exhibits an out-of-plane saturation magnetization Ms┴=1690 emu/cm3 (15 times the bulk magnetization value) and in-plane Ms//=216 emu/cm3, corresponding to 3.3 and 0.43μB/Mn at. Giant magnetization is attributed to uncompensated Mn moments located at the intergrain boundaries. No surface anisotropy is expected due to the high anisotropy constant of Mn3Ga. Ms decreases as the film thickness increases due to grain growth and subsequent decrease in intergrain boundary surface area. Magnetic M(H) loops remain anhysteretic for thickness 10 nm, corresponding to an average grain size <50 nm, suggesting that nanocrystals are magnetic monodomains. Films with thickness >10 nm exhibit coercivity between 9 and 19 kOe and Ms from 25 to 72 emu/cm3 with a residual weak ferromagnetic component. Hysteretic M(H) loops are the signature of the existence of magnetic multidomain grains with average grain size >50 nm. All films exhibit higher out-of- plane Ms than in-plane magnetization.