Nextel 610 alumina fiber tows were heat-treated at 1100 degrees C-1500 degrees C for 1 to 100h in air. Tensile strengths and Weibull moduli were measured for 30 filaments after each heat-treatment. 3-D grain size and orientation distributions were described using oblate ellipsoids. The number of grains in a 1inch gauge length and grains with the largest major and minor ellipsoid-axes were determined from these distributions. The grain with the largest K-EFF for mixed-mode fracture was also determined, using the maximum energy release rate criteria from grain-size and orientation distributions. Grain-size dependence of tensile strength and Weibull modulus was evaluated. Strength had no obvious dependence on grain size for fibers with average major-axes smaller than 0.25m. For fibers with larger grains, grain-size dependence may involve flaws originating from clumps of grains, rather than a single grain. Possible relationships between strength and grain-size and other causes of strength degradation after heat-treatment are discussed.