The effects of thermomagnetic pretreatments on the magnetism and mechanical properties of Ni50.3Mn28.7Ga21 single crystal were studied. Two samples A and B were heated to a temperature above their Curie temperature （TC） and then cooled down to room temperature via different cooling processes. An external magnetic field was applied during cooling of sample A from T>TC to room temperature. For sample B, the same magnetic field was applied at T below its TC but higher than its austenite to martensitic transformation temperature during cooling. The reversible strain and change in magnetization （ΔM） measured at room temperature for sample A are much smaller than those for sample B. It was supposed that with the appearance of the magnetic field during the transformation from austenite to martensitic phase, the field tends to induce a favorable single martensitic domain in sample B, leading to the greater reversible strain and ΔM. However, preferential magnetic domains may develop in sample A by applying the same magnetic field during the transition from paramagnetism to ferromagnetism, leading to the different alignments of martensitic domains from those in sample B. This might be the reason for the smaller reversible strain and ΔM in sample A.