The composition of a machineable glass ceramic containing ZrO2 was optimized using machineability testing. The effects ofK2O, CaO, MgO and Al2O3 on the machineablity of the glass ceramic were studied and the results analyzed in terms of the relationship between crystallization 0f a mica phase and composition of the bulk. The XRD result shows that the glass ceramic after optimization contains a large amount of tetragonal ZrO2 and mica phases.