The influence of heat treatment with different cooling rates on phase transition behaviors and magnetocaloric effect is systematically studied.Difference in atomic order is induced by changing cooling rates,where ordered phase is obtained in the furnace cooled(FC)sample while disordered phase is reserved in the water quenched(WQ)sample.The coupled magneto-structural transition is detected in both samples but the characteristic temperature significantly shifts to lower temperatures with increasing atomic order.Giant magnetic entropy change(ΔS_(mag))derived from magnetic field induced martensitic transformation is confirmed for both samples,and can be remarkably enhanced by the atomic ordering.The largestΔS_(mag) of 20.9J/(kg·K)is obtained at 307.5Kunder 5Tin the FC sample. The influence of heat treatment with different cooling rates on phase transition behaviors and magnetocaloric effect is systematically studied. Difference in atomic order is induced by changing cooling rates, where ordered phase is obtained in the furnace cooled (FC) sample while disordered phase is reserved in the water quenched (WQ) sample. The coupled magneto-structural transition is detected in both samples but the characteristic temperature significantly shifts to the lower temperatures with increasing atomic order. Giant magnetic entropy change (ΔS_ (mag)) derived from magnetic field induced martensitic transformation is confirmed that both samples, and can be remarkably enhanced by the atomic ordering. The largest ΔS_ (mag) of 20.9J / (kg · K) is obtained at 307.5K 5Tin the FC sample.