Role of ternary solute element Nb on the C40→C11b phase transformation of the MoSi2 phase in MoSi2/Mo5Si3 eutectic alloys which occurs during heat treatment is discussed in this paper. In the case of stoichiometric MoSi2 a mixture of C40 and C11b phases can be prepared by low-pressure plasma spraying method, in which phase transformation from the metastable C40 to stable C11b is recognized during heating at around 800°C. It is found that the MoSi2 phase in an as-cast eutectic MoSi2/Mo5Si3 alloy with 8.5 at%Nb consist only of metastable C40 phase. The transformation from C40 to C11b in this alloy does not occur by heating up to 1200°C but a portion transforms into C11b by heating at 1400°C. It is shown that the Nb concentration in C40 phase is higher than that in C11b phase in this alloy, which must be relevant to the observed transformation behavior.