The magnetic phase transitions of all-d metal Heusler type model, Ni2MnZ (Z = Ti, Sc and V), are investigated by using the double-time Green's Function Method within the random phase approximation. We assume that the magnetic moments of Ni, Mn, and Z in Ni2MnZ which crystallized in L21 structure is localized on their atom and the next-nearest neighbor interactions between Mn atoms are negative. While the Z atoms give only a small contribution to the total magnetic moment, it is emphasized that the importance of existence of antiferromagnetic interaction between Z atom and other atoms in observing first-order phase transitions. Ni2MnZ shows a ferromagnetic state depending the values of the Hamiltonian parameters. If any one of these interactions have negative value and the system is subject to an external magnetic field, it is observed that the magnetization curves exhibit to the first-order phase transitions.