The post annealing temperature and Co thickness dependence of tunnel magnetoresistance (TMR) ratio was investigated for L10-MnGa/Co(tCo)/MgO/CoFeB perpendicular magnetic tunnel junctions (MTJs). The MnGa/MgO interface optimization by Co insertion was shown to be an effective way for improving the TMR ratio. The TMR value increases with the annealing temperature (Ta), and exhibits maxima at 325 °C for tCo = 1.0 - 5.0 nm. Thermal annealing process improves the structure of MTJs, but also causes elements diffusion. In this work, the annealing effect on MTJs with different Co thicknesses was discussed in detail along with a fast annealing method.