Anisotropic magnetoresistance (AMR) effects in Cox(MnyGa1- y)100- x Heusler alloy thin films epitaxially grown on a MgO single-crystal substrate have been investigated by changing their composition in the ranges of x = 44.4-59.2 at. % and y = 0.44-0.55. Negative AMR ratios were observed at measurement temperatures from 5 to 300 K for all the films, indicating that Co2MnGa films have possible half-metallicity. The AMR ratio changed sensitively depending on the composition of the films. In the case of the film with y = 0.44, the AMR ratio peaked at around x = 50 at. %. In addition, the amplitude of the AMR ratio increased with decreasing y when x was approximately 50 at. %. As a notable result, large negative AMR ratios of -2.32% at 5 K and -0.82% at 300 K, whose amplitude is more than four times that of Co2MnSi films, were obtained for the Co-Mn-Ga films. These results suggest that the Co2MnGa films have a high spin polarization due to half-metallicity.