We prepared a polycrystalline sample of orthorhombic La0.7Ca0.3Mn0.85Ni0.15O3, and then studied its critical behavior. Based on the Landau phase-transition theory and Banerjee's criteria, we have found the sample undergoing a second-order magnetic phase transition. This continuous transition is characterized by critical parameters of TC = 170 K, β = 0.320 ± 0.009, γ = 0.990 ± 0.082, and δ = 4.09 ± 0.17 determined from modified Arrott plots. With these values, the magnetization-field-temperature (M-H-T) behaviors of La0.7Ca0.3Mn0.85Ni0.15O3 below and above TC are well described by an equation of state M(H,ε)=εβf±(H/εβ+γ), where ε=(T−TC)/TC, f+ for T > TC and f− for T < TC. Having compared to the theoretical models, the critical-exponent values determined in our case are close to those expected for the 3D Ising model and mean-field theory. This indicates an existence of ferromagnetic short-range interactions in the system below TC. The nature of the transformation from first-order to second-order phase transitions related to the Ni doping is discussed in detail.