Abstract: The electronic structure, magnetic moment, crystal structure parameters and characteristics of microscopic magnetism of Ni2MnIn Heusler alloy and tetragonal martensite(c/a=1.429) were studied theoretically via ab-initio pseudo-potential calculation based on density functional theory. Analyses of the bands, atomic magnetic moments and partial electron density of state (PDOS) showed that no obvious change is found in atomic orbital magnetic moment, unit cell's orbital magnetic moment and cell volume of each and every element in the Ni2MnIn Heusler alloy and tetragonal martensite; and the spin polarization is found in both phases as above. Calculated results showed that in the Ni2MnIn phase, the atomic orbital magnetic moment of Mn dominates the unit cell's magnetic moment, i.e., about 85% of the total magnetic moments of unit cells. The contribution of Ni to the total cell's magnetic moments is about 15%. As to the In atoms, they show a weak diamagnetism in both phases. In particular, the phase transformation of tetragonal martensite results in the decrease in Fermi energy of Ni2MnIn unit cells by 0.495eV. The theoretically calculated results were compared with other theoretic values.