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The magnetocaloric effect (MCE) on Ni_2−xPt_xMnGa (x = 0.2, 0.3 and 1.0) shape memory Heusler alloys around martensite phase transition temperature (T_M) is investigated by varying chemical pressure (Pt concentration). The magnetic entropy change (ΔS_M) decreases with increasing chemical pressure for various external applied magnetic fields up to 3 T, and the width of thermal hysteresis increases with the increases of Pt concentration. The effect of hydrostatic pressure on both T_M and ΔS_M for Ni_1.8Pt_0.2MnGa is also investigated. We observed that the application of hydrostatic pressure increases T_M (3.5 K/GPa) and stabilizes the martensite phase. The maximum magnetic entropy change (ΔS_max) of 9.31 J Kg⁻¹ K^-1is observed for a field change of 9 T at ambient pressure for Ni_1.8Pt_0.2MnGa. Further, the application of external pressure leads to the decrease of ΔS_max to 5.52 J Kg⁻¹ K⁻¹ at 0.91 GPa.