In the as-cast structures of Fe-12%Mn-7%Ni-3.5%C alloy, graphite particles appeared with carbide which is metastable phase. According to the result of the previous experiment that this carbide could be decomposed through heat-treatment for 70hrs at 1100℃, in this study, Mn, carbide stabilizer, was reduced to 5 through 10% and Si, graphite stabilizer, was added to 0.5 through 3.0%. Using optical microscope, the amount of carbide and graphite was observed. In order to decompose the carbide, specimens were heat treated at 1100℃ and then water quenched. The results obtained in this study are as follows. 1. With increasing Mn content and decreasing Si content, the amount of carbide increased and the amount of graphite and graphite nodule count decreased. 2. Carbide was decomposed to the austenite and temper carbon through heat-treatment at 1100℃. The nodule count of temper carbon increased with increasing Si content and decreasing Mn content. 12∼15hrs was required for the complete decomposition of the carbide. 3. The hardness (HR_B) increased as carbide increased, but after carbide was completely decomposed the hardness increased in proportion to Mn and Si content.