The effect of sintering conditions on the sinterability for silicon nitride has been studied by many in-vestigators. However the effect of sintering conditions on the machinability which is the major barrier to the field applications of the ceramic components has not been fully studied. In this study the sintering con-ditions such as temperature gas pressure and time in silicon nitride were varied. The physical and mechan-ical properties of the gas pressure sintered (GPS) silicon nitride were measured. The optimum mi-crostructure of silicon nitride with the excellent machinability was investigated by MFG(magnetic-fluid grinding) technique. An attempt was made to figure out how the mechanical properties influence upon the machinability of silicon nitride ball.