We computed the electronic structure, elastic moduli, vibrational properties, and Ni2TiGa and Ni2ScGa alloys in the cubic L21 structure. The obtained equilibrium lattice constants of these alloys are in good agreement with available data. In cubic systems, there are three independent elastic constants, namely C11, C12 and C44. We calculated elastic constants in L21 structure for Ni2TiGa and Ni2ScGa using the energy-strain method. The electronic band structure, total and partial density of states for these alloys were investigated within density functional theory using the plane-wave pseudopotential method implemented in Quantum-Espresso program package. From band structure, total and projected density of states, we observed metallic characters of these compounds. The electronic calculation indicate that the predominant contributions of the density of states at Fermi level come from the Ni 3d states and Sc 3d states for Ni2TiGa, Ni 3d states and Sc 3d states for Ni2ScGa. The computed density of states at Fermi energy are 2.22 states/eV Cell for Ni2TiGa, 0.76 states/eV Cell for Ni2ScGa. The vibrational properties were obtained using a linear response in the framework at the density functional perturbation theory. For the alloys, the results show that the L21 phase is unstable since the phonon calculations have imagine modes.