Plastic deformation behavior and the dominant deformation mode in ternary (Nb1-xMox)Si2 (x=0-0.85) single crystals with the C40 structure were examined focusing on the phase stability between the C40 and C11b structures. Temperature dependence of CRSS for the (0001)<2-1-10] slip is strongly influenced by the Mo concentration accompanied by the change in the dissociation mode from the co-planar-type to the synchroshear-type. In the ternary crystals with x=0-0.30, anomalous strengthening clearly appeared, and the peak stress increased up to 1600¢ªC with an increase in the Mo content. Mo atoms may gather and form a dragging atmosphere around 1/3<2-1-10] dislocations containing a superlattice intrinsic stacking fault (SISF), resulting in anomalous strengthening. In contrast, in crystals with x=0.60-0.85, a high CRSS was obtained with fracture in a brittle manner at low temperatures, and the stress decreased remarkably with an increase in the test temperature. HRTEM observation showed that the synchroshear-type dissociation of each super-partial dislocation, 1/6<2-1-10], occurred on the neighboring slip planes, accompanied by a much wider separation distance of SF than that in the ternary crystals with a low Mo content. This change in plastic deformation behavior may be closely related to the decrease in phase stability of the C40 structure with the addition of the C11b-stabilized element, Mo. The phase stability can be represented by the ratio of (h/b), where h is the (0003) spacing and b is the amplitude of 1/3<2 0]. As the (h/b) ratio increases with the addition of Mo, the dissociation mode changes from the co-planar-type to the synchroshear-type at a ratio of 0.465.