The crystal structure and magnetic structure of hard magnetic alloy Mn1.074Al0.871C0.055 have been investigated by neutron diffraction technique. Mn1.074Al0.871C0.055 has body center tetragonal structure. The refinement of neutron diffraction data shows that 0.992 Mn and 0.008 C occupy (0,0,0) site and 0.082 Mn, 0.871 Al and 0.047 C occupy(1/2,1/2,1/2)site. The magnetic moments of manganese atoms in the different sites are alongc-axis pointing in opposite directions. In comparison of these results with that of binary alloy Mn1.09Al0.91, the significant improvement of magnetic and mechanical properties in ternary alloy Mn1.074Al0.871C0.055 can be explained.