Abstract: In order to investigate the effects of In-substitution for partial Ge on magneto-structural transition and magnetocaloric effect (MCE), Mn1Co1Ge1 and Mn1Co1Ge0.99In0.01 alloys were prepared by arc-melting method, and phase composition, phase transition, MCE and magnetization isotherms of Mn1Co1Ge0.99In0.01 alloy obtained by warming and cooling modes were studied. The results show that In-substitution for partial Ge would not lead to impurity phase, but could turn the second-order magnetic transition to first-order magneto-structural transition. Mn1Co1Ge0.99In0.01 alloy owned different magnetism hysteresis behaviors on different phase transformation directions. In warning mode, the maximal magnetization entropy of Mn1Co1Ge0.99In0.01 alloy at magnetic intensigy of 5, 7 T were 52.2, 64.1 J·kg-1·K-1, respectively; in cooling mode, those were 40.8, 50.9 J·kg-1·K-1, respectively. The refrigeration capacity of Mn1Co1Ge0.99In0.01 alloy at magnetic intensity of 7 T was 477 J·kg-1 in warning mode, and that was 264.4 J·kg-1 in cooling mode.