To investigate the effect of vacancy concentrations on mechanical properties of single crystal gamma-TiAl, the uniaxial tension tests of gamma-TiAl alloy models at different vacancy based on molecular dynamics simulation was performed. The results shows that obvious brittle behavior in stress-strain curve and ultimate stress decreases nonlinearly with increasing vacancy concentrations can be got. When the atom near vacancy is changed, potential energy reaches to maximum, and decreases with the increasing of vacancy concentrations. In the process of tension tests, vacancy gradually is evolved to the hole, and then the hole is evolved to micro cracks. Finally the micro crack extended until the material fractures. The form of crack propagation is different at different vacancy concentrations.