The hot deformation behavior and deformation mechanism of TiAl/Ti2AlNb diffusion bonded (DB) joint in the temperature range of 900 degrees C-1000 degrees C and strain rate range of 0.001 s(-1)similar to 0.1s(-1) were investigated by isothermal compression tests. The deformation may be related to the coordination deformation of the two alloys, with the deformation of Ti2AlNb section coming firstly, followed by TiAl due to the existence of the DB interface. The flow behavior of DB joint can be expressed by the hyperbolic sine constitutive equation, and the calculated deformation activation energy Q is 343.5 kJ/mol. The Q value is almost equal to lattice diffusion coefficient (265-331 kJ/mol) in O + B2 Ti2AlNb alloy, which suggests the deformation mechanism could be controlled by lattice diffusion. Microstructure observation shows that, When deformed at 900 degrees C-0.1s(-1), the deformation mechanism is grain boundary sliding accommodated by O-DRX in Ti2AlNb, gamma-DRX in TiAl, deformation twin and beta ->alpha(2) phase transformation. While the deformation mechanism is dislocation slip accommodated by beta+O ->epsilon phase transformation , epsilon-DRX, beta-DRX in Ti2AlNb and deformation twin when deformed at 1000 degrees C-0.001s(-1). Besides, the 3D processing map of TiAl/Ti2AlNb DB joint were obtained with deformation temperature and strain rate. There are no flow instability regions of DB joint and the optimum processing parameters were determined to be 1000 degrees C-0.001s(-1), favoring development of a new structural component.