Isothermal compression of the Mo-Nb single crystals was conducted on a Gleeble-3800 thermal simulator at the deformation temperature range of 1100-1300℃with strain rates ranging from 0.001 to 10s-1 and height reduction of 50%, true strain is 0.7. The results show that the deformation temperature and strain rates affected the flow stress during the thermal deformation of Mo-Nb single crystals significantly. The true stree-strain curves exhibit a peak flow stress, flow softening and steady flow behavior. The constents of Mo-Nb single crystals, such as the activation energy Q, the stress exponent n, are calculated. The constitutive equation is estabilished based on a hyperbolic-sine equations of Arrhenius type.