Ultrafine HfB2 powders were synthesized by the combination of borothermal reduction of HfO2 and solid solution of 5 mol% TiB2 or 5 mol% TaB2, prototypically, (Hf0.95Ti0.05)B-2 and (Hf0.95Ta0.05)B-2. The influence of substitution on the particle growth, high-temperature stability, densification, microstructure, and mechanical properties of HfB2 was investigated. Results showed that the particle sizes of HfB2, (Hf0.95Ti0.05)B-2 and (Hf0.95Ta0.05)B-2 powders prepared by borothermal reduction at 1500 degrees C were 1.73, 0.87, and 0.21 mu m, respectively. The substitution of TaB2 led to a greater decrease in particles size than TiB2. After heat treatment at 1800 degrees C, the particle sizes of HfB2, (Hf0.95Ti0.05)B-2 and (Hf0.95Ta0.05)B-2 powders increased to 2.60, 1.59, and 0.32 mu m, respectively, indicative of the good high-temperature stability of TaB2-substituted HfB2. The relative densities of HfB2, (Hf0.95Ti0.05)B-2 and (Hf0.95Ta0.05)B-2 ceramics after spark plasma sintering at 2000 degrees C were 76.1%, 85.2% and 99.8%, respectively. The fully dense (Hf0.95Ta0.05)B-2 ceramics with fine microstructure showed comparably high Vickers hardness of 21.1 GPa combined with flexural strength of 521.2 MPa. It was proved that the solid solution of TaB2 could effectively inhibit the grain growth of HfB2 powders, and improve the densification, microstructure, and mechanical properties of HfB2 ceramics.