Mössbauer measurements have been carried out on Ni-substituted Mn-ferrites, Mn1-xNixFe2O4 (x = 0, 0.25, 0.50, 0.75 and 1.0), at room temperature. The cation distribution at tetrahedral (A) and octahedral (B) sites and its effect on Mössbauer parameters is studied. Mn2+ ions are found to occupy sites A and B both, while Ni2+ ions occupy site B only. It has been observed that the internal magnetic fields (Heff) for all B sites increase and it remains almost constant for site A with increasing Ni concentration. The line widths of the most of B sites are decreased and no significant change is observed in isomer shift with increasing Ni concentration.