Abstract Fracture toughness ( K IC), an important property of brittle materials, can be determined by indentation cracking tests. This paper reports on an investigation of the fracture toughness of a directionally solidified Al 3Nb–Nb 2Al–AlNbNi ternary eutectic, based on the Vickers indentation test applied to longitudinal and transverse sections of its microstructure. The measurements were taken using indentation loads varying from 2.45 to 24.5 N. Correlations between the resulting crack parameters and indentation load were evaluated using Palmqvist and half-penny cracks models. In the range of indentation loads studied, the results suggested that the Palmqvist model provided a better fit to the experimental data. Fracture toughness was calculated using equations developed for Palmqvist crack mode. The indentation fracture toughness values for longitudinal and transverse sections are in the range of 2.82–3.05 MPa m 1/2 and 2.98–3.59 MPa m 1/2, respectively. It was found that the addition of Ni and incorporation of a third phase to the Al 3Nb–Nb 2Al eutectic improved fracture toughness of this in situ composite material.