Pure and chromium (Cr)-doped sapphire microfibers (100-160 μm in diameter) have been tested under constant tensile stress rates. KIC has been evaluated by means of fractographic measurements to determine the size of the critical flaw and the size of the subcritical crack growth (SCG) region surrounding the originating flaw. For high-temperature conditions a simple relationship between the size of the mirror and the size of the SCG zones is also presented. This information allows KIC to be determined by simply measuring the size of the mirror zone. The failure stress of sapphire fibers is highest at room temperature, 2.6 GPa for pure sapphire, diminishing to 0.5 GPa at 1400°C. A decrease in strength is also observed in Cr-doped fibers, 1.7 GPa at room temperature to 0.4 GPa at 1400°C. A similar decrease in KIC is also observed for Cr-doped fibers, 0.38 MPa·m 0.5 at room temperature to 1.4 MPa·m0.5 at 1400°C. Advantages of the procedure suggested for KIC determination include no need for artificially introduced precracks, repeatability, applicability to high-temperature conditions, and good agreement with literature results. Additionally, the short evaluation times and simple measurements and calculations involved are well suited for industrial quality control settings. © 2008 The American Ceramic Society.