The mechanical properties of fluorophlogopite mica-based glass-ceramics, such as fracture toughness, flexural strength, and machinability, were investigated, It has been shown that toughening increments in the glass-ceramics occurred by crack deflection and branching by crystals with a high aspect ratio. All the glass-ceramics heat treated at 1000 degrees-1150 degrees C exhibited a higher fracture toughness of 1.2-2.2 MPa . m(1/2) as compared to 0.8 MPa . m(1/2) for the parent glass, and showed average flexural strengths of 140-160 MPa. It has been suggested that (H/K-Ic)(2) of the mica glass-ceramic be used to estimate the machinability, because it decreases linearly with machinability,