The hot deformation behavior of Mo-Nb single crystals has been characterized through strain-stress curves and processing map in the temperature range of 1100-1300℃ and strain rates range of 0.001-10 s-1. The results indicate that the flow stress was affected significantly by deformation temperature and strain rate during the hot deformation of Mo-Nb single crystals. Base on the processing map, the optimum condition for hot deformation of Mo-Nb single crystals is the region of high temperatures (>1190℃) and high strain rates (>3.16s-1). Through observation of microstructure of deformed specimens at different deforming conditions, lots of orthogonal cracks, which result from the movement of dislocations along with the glide planes, were observed in specimens deformed at 1100℃/0.01s-1 and 1150℃/10s-1. Less cracks can be observed in specimens deformed at 1250℃/0.01s-1, and few cracks were found in specimens deformed at 1300℃/10s-1. The XRD result shows that the specimen deformed at 1300℃/10s-1 still retains single crystal structure, indicating that the specimen was not undergone instability of flow during hot deformation at the deformation condition of 1300℃/10s-1, which are in good agreement with the results of Mo-Nb single crystals predicted from processing map.