Abstract: The filled skutterudite compound PrFe₄P₁₂ exhibits anomalously phase transition and heavy Fermion (HF) state below 6.5 K. In order to investigate effects of 3d-electron on the phase transition and HF state, samples of Pr(Fe_1-xCo_x)₄P₁₂ were synthesized to observe evolution of f electron state by specific heat, magnetic susceptibility measurement and inelastic neutron scattering. The experiment results indicate that the properties of PrFe₄P₁₂ are very sensitive to Co doping. The ordered state and HF state are suppressed by only 3% Co substitution, and the well-localized nature of f electrons appears in the Co-substituted system.