We have investigated the Fe/Cr doping effects on the magnetic property of a charge-ordered antiferromagnetic manganite La0.4Ca0.6MnO3. Magnetic measurements reveal interesting doping effects. While all the Fe-doped samples still have antiferromagnetic ground state, strong ferromagnetic tendency can be seen in the Cr-doped samples. Meanwhile, Cr-doped samples show clear metallic transport behavior, indicating an inherent double exchange mechanism responsible for the ferromagnetic metallic state. We thus propose that the magnetic exchange interaction between Mn and dopants, and the d-orbital electronic structure of Cr/Fe are essential for the distinct doping effect of Fe and Cr.