The composition of machinable glass ceramics in the SiO2 Al2O3 MgO K2O B2O3 Li2O ZnO F system has been studied in order to decrease their melting temperature and to increase their strength at the same time. The results show that the melting temperature and the crystallizing temperature are decreased because of adding low melting point oxides Li2O and ZnO. ZnO can expande the forming area of glass and make the glass with higher content of Li2O transparent. At the optimizing composition of SiO2, Al2O3 and MgO, cordierite is formed among mica crystals contacted each other, and the glass phase is decreased. Thus the mean free path between crystals is decreased, which results in the high strength of the machinable glass ceramic.