In this study we have investigated the effects of Mm, MmH and MmH₂ on the matrix development in cast iron, The conclusive summary is as follows : The spheroidal graphite was observed when 0.5wt.% or more of mischmetal was added and the matrix was of ledeburite structure, but hull`s eye structure was not observed. On the other hand, the bulls eye structure was observed when 0.25wt.% of MmH, or 0.25wt.% to 0.5wt.% of MmH₂was added. Above limit of the additives, the matrix changed into ledeburite structure. As the hydrogen content of mischmetal compound increased from MmH, the range of additives to obtain bulls eye structure expanded. This reveals the significant effect of mischmetal hydride on matrix development in cast iron and the possibility of practical use of the additives.