This study is focused on the effect of the sintering additive on the microstructure and properties of HfB2 composites reinforced with SiC Hi–Nicalon fibers. The materials were spark plasma sintered with addition of 5–10 vol% of Si3N4 or ZrSi2. The fibers morphology was examined using scanning and transmission electron microscopy and remarkable differences were observed depending on the additive. Local hardness and Young's modulus were measured by nanoindentation to investigate the fiber mechanical behavior. The introduction of 20 vol% SiC short fibers into the HfB2 matrix increased the fracture toughness up to 5.4 MPa m1/2 for the material sintered with ZrSi2. The room temperature flexural strength of the materials containing fibers slightly decreased when ZrSi2 was used, but with the addition of Si3N4 an improvement of more than the 40%, as compared to the unreinforced material was observed.