Zirconium diboride and hafnium diboride were fabricated by hot-pressing at 180OoC and 120,000 psi. Bend strengths were measured on the fully dense materials from 25' to 140OoC in an argon atmosphere. These diboride compounds do not exhibit any gross plastic flow in the temperature range studied. The bend strengths go through a maximum between 700' and 1000°C and vary from 39,000 to 68,000 psi for HfB2 and 30,000 to 56,000 psi for ZrBz. The maxima in strength correspond to maxima in the fraction of transgranular fracture. The bend strength and room-temperature elastic modulus measurements Were combined with available thermal conductivity and expansion data to calculate thermal stress resistance parameters. Under steady-state heat flow conditions, the calculated thermal stress resistance parameters of the borides are higher than those calculated for other refractory compounds.