Fabrication, microstructure and properties of in situ formation of strengthened eutectic composites, consisting of a matrix (B₄C, and B₄C–NbB₂ composite) and also containing regularly distributed whiskers of NbB₂ by spark plasma sintering (SPS) is reported. It has shown that pressure and the temperature at which pressure is decreased during SPS may be used as a means of controlling the eutectic formation and composites microstructure. The microstructure-property relations were determined for composites of NbB₂–B₄C eutectic composition as a function of NbB₂ content, eutectic inclusion rod size and NbB₂–NbB₂ spacing distance. It was found that the composites with the eutectic composition of 30–35 mol.% NbB₂ obtained by SPS exhibit a high Vickers hardness (26–27 GPa) and a maximum in indentation fracture toughness (~7 MPa·m^1/2) in case of interlamelar spacing distance of 0.8 ± 0.4 µm.