A study has been made to investigate the boundary sliding and its accommodation mode with respect to the variation of $alpha$$_2$/$eta$ volume fraction during superplastic deformation of two-phase Ti$_3$Al-xNb intermetallics. Step strain rate and load relaxation tests have been performed at 950, 970 and 99$0^{circ}C$ to obtain the flow stress curves and to analyze the deformation characteristics by the theory of inelastic deformation. The results show that the grain matrix deformation and boundary sliding of the three intermetallics containing 21, 50 and 77% in $eta$ volume fractions are well described by the plastic deformation and viscous flow equations. Due to the equal accommodation of both $a^2$ and $eta$ phases, the accommodation modes for fine-grained materials are in good agreement with the iso-strain rate models. The sliding resistance analyzed for the different boundaries is the lowest in the $alpha$$_2$/$alpha$$_2$ boundary, and increases in the order of $alpha$$_2$/$alpha$$_2$<< $alpha$$_2$/$eta$ = $eta$/$eta$, which plays an important role in controlling the superplasticity of the alloys with the various $alpha$$_2$/$eta$ phase ratio.