Mn3GaC is antiferromagnetic (AFM) at low temperatures and ferromagnetic (FM) above Tt (∼ 160 K). The magnetization of Mn3Ga1-xAlxC with x ≤ 0.08 has been measured under pressure (p) with a special interest in the p dependence of magnetic properties in this system. It is found that the pressure enhances the magnetic moment of Mn μFM in the FM phase and changes the AFM to a new magnetic phase with spontaneous magnetization. The forced magnetostriction of Mn3Ga1-x, AlxC with x = 0.015 and 0.07 indicates μFM < μNEW PHASE < μAFM.