Giant magnetocaloric effect based on first order phase transformation has been investigated extensively recently. A considerable magnetic entropy change has been found in single crystal Ni52.6Mn23.1Ga24.3, Ni53Mn22Ga25 and polycrystal Ni(51.5)Mn(22.7)GE(25.8). This change originated from a sharp magnetization jump caused by the martensitic-austenitic structure transition on heating. in this paper, magnetocaloric effect in the alloys Ni54.9Mn20.5Ga24.6 is studied. The Curie point temperature Tc of the alloy is adjusted to the vicinity of martensitic transition temperature Tm. The concurrence of martensitic structure transition and magnetic phase transition enhance the magnetocaloric effect in these alloys. The martensitic structure transition effect on the magnetic properties of the alloys is investigated. The character of magnetocaloric effect during the transition from the austenitic to martensitic state is discussed.