Fe - C - Si - Mn alloys are used as blades in hydroelectric generators contained network structures after some heat treatments. It is found by metalograph and TEM analysis that annealed network structure is pearlite with very fine slice while the normalized is martensite or bainite. Experimental results together with theoretical analysis prove that the network structures are resulted from segregations of carbon and manganese atoms in the interdendritic areas, higher heat treatment temperature or longer soaking time can enlarge the grid size and finally eliminate the network structure. The network structure has some effects on the strength, and the smaller the grid size, the higher the strength.