[en] The effect of B-addition on the microstructure and fracture toughness of (Mo_0_._8_5Nb_0_._1_5)Si_2 crystals with an oriented lamellar microstructure was investigated. B-addition led to an increase in the volume fraction of the C11_b phase, which possesses different orientation relationship from that of the fine lamellae, and a reduction in their precipitation rate. The fracture toughness of the B-added crystal with the varied microstructure exhibited a value more than 4.0 MPa m"1"/"2, that was significantly higher than that of the ternary crystal.