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In this paper, the magnetocaloric properties, phase transition and critical behavior of Mn5Ge3-xZnx (x=0.1,0.2 and 0.3) compounds fabricated by the arc-melting method were investigated. The Curie temperature and the magnetic entropy change of Mn5Ge3-xZnx alloys decrease with doping of Zn. The normalized curves of universal behavior further demonstrate the feature of second-order transition in the Mn5Ge3-xZnx alloys, consistent with the results of the Landau coefficient and Banerjee criterion analysis. To explore the properties near the phase transition, we investigated the critical behavior of this series using modified Arrott plots, Kouvel-Fisher method and critical isotherm curves. The critical exponents β, γ and δ were calculated in accordance with the Widom scaling relation, δ=1+γ/β , and magnetic state equation, M(H,ε)=εβf±(H/εβ+γ) , demonstrating the experimentally obtained exponents are accurate. The spontaneous magnetization derived from the isothermal entropy change calculated from Maxwell equation, is in agreement with the spontaneous magnetization calculated from the H/M-M2 curves, proving the validity of calculating the spontaneous magnetization from the |∆SM |-M2 curves.