Silicon nitrides containing various compositions of Yb2O3 were investigated. The effects of post heat-treatment on the microstructure, grain boundary phases and high temperature strength were studied. The average grain size, and Yb:Si ratio in the amorphous pocket increased with the amount of Yb2O3 additives. The high temperature strength was substantially increased after post heat-treatment. This was attributed to the crystallized grain boundary phases post heat-treatment.