We report on the magnetic properties and magnetocaloric effects of Mn5Ge3-xGax compounds with x=0.1, 0.2,0.3, 0.4, 0.6 and 0.9. All samples crystallize in the hexagonal Mn5Si3-type structure with space group P63/mcm and order ferromagnetically. The Curie temperature of these compounds decreases with increasing x, from 306K (x=0.1) to 274K (x=0.9). The average Mn magnetic moments increases with increasing Ga content, reaching a maximum value at x=0.6. The magnetic entropy changes in these compounds are determined from the temperature and field dependence of the magnetization using the thermodynamic Maxwell relation. The Ga substitution has two kinds of influence on the magnetocaloric effect (MCE) of Mn5Ge3. One is that the magnitude of the magnetic entropy change decreases, the other is that the MCE peak becomes broadened.