Crystallographic, thermal strain, and magnetic properties of Ni 50-xCoxMn38.5Ga18.5 (0≤x≤9), were investigated across the martensite transition temperature TM and the reverse martensite transition temperature TR at atmospheric pressure. These transition temperarures increased gradually with increasing Co component x. Moreover, temperature hysteresis in the thermal cycles of the magnetization across the TR and TM, became larger with increasing x. Wide temperature hysteresis of TM -TR = 65 K was observed in the thermal strain measurement. The metamagnetic transition from the paramagnetic martensite phase to the ferromagnetic austenite phase appeared between 330 K and 390 K. The magnetostriction of this alloy was 0.11% at 350 K under atomospheric pressure. This value of the magnetostriction is larger than that of TbDyFe single crystal under atomospheric pressure. The samples of Ni50-xCoxMn38.5Ga18.5 in this study were polycrystal, and it is easy for processing and handling. Moreover, the magnetostriction effect occurred at a temperature between room temperature and 390 K, therefore it is useful in the high temperature region, e.g. the engine room of the motorcar.