The influence of pore size on the toughness of Fe-Mo-Cu-C sintered and carburized components was investigated. The charpy impact value of the component made from Fe-0.4%Mo diffusion-alloyed steel powder mixed with 2%Cu powder and 0.3% graphite powder significantly increased with a decrease in pore size. The highest impact value of 17 J/cm2 was obtained at the average pore size of 12 µm. This value was 25% higher than that of the sintered and carburized component made from conventional Fe- 4%Ni-1.5%Cu-0.5%Mo alloyed steel powder called 4Ni even though the component was made from a Ni-free alloyed steel powder. The mechanism of the improvement of the impact value by pore size refinement and the influence of microstructural difference between the components made from Fe- 0.4%Mo diffusion-alloyed steel powder and 4Ni on toughness is discussed based on optical microstructural observation and crystal orientation analysis data.