The matrix changes and graphite formation in high manganese cast iron (Fe-12Mn-3.5C) are studied with increasing nickel and silicon content. Also, the decomposition of carbides and graphite precipitation are studied by adequate heat treatment. The results obtained in this work are as follows. 1. In high manganese cast iron, fine flakes graphite appeared by adding 5wt% nickel and A-type flakes graphite can be obtained by adding 7wt% nickel. 2. Nodular graphite are obtained by graphite spheroidizing treatment with same melt. 3. In high manganese cast iron containing 7wt% nickel, full austenitic matrix with nodular graphite can be achieved by water quenching after 10hours` solution heat treatment at 1050℃ in case of containing 2.0wt% silicon, and 6hours` at the same temperature in case of containing 2.5wt% silicon.