We report the structural studies on Mn excess and Ga deficient Ni2Mn1+zGa1-z specimens with z= 0, 0.05, 0.1, 0.15, 0.2 and 0.25. The crystal structure at room temperature was determined by the x-ray diffraction (XRD). Rietveld analysis has been performed to obtain the lattice parameters. For z= 0, 0.05 and 0.1, a cubic austenitic phase is observed. For 0.15≤ z ≤0.25, a tetragonal martensitic phase is obtained, whose lattice constant c increases and a decreases linearly with increasing z following Vegard’s law. Phase coexistence is observed for z= 0.15, confirming the first-order nature of the martensitic transition.