Diffusion is an important particle behavior in granular flow. Although granular diffusion has been studied for decades, the diffusion of size bidisperse particles has not been well understood. In this paper, discrete element method simulations with the Lees-Edwards boundary condition are performed to quantify the relation between the diffusion coefficient ($D$) and flow parameters for size bidisperse spheres in dense granular flow. The influences of the shear rate ($\dot{\gamma }$), the solids fraction $\left(f\right)$, and the diameter ratio $\left(D_{LS}\right)$ of particles on diffusion are studied. The effects of the friction coefficient (μ) and the restitution coefficient ($e$) are also investigated. The results indicate that while small particles diffuse faster than large particles in a binary system the volume weighted average diffusion coefficient is proportional to the shear rate and the square of the volume weighted average particle diameter, $d^2$, and it is inversely proportional to the solids fraction. The quantified relation is given as $D=k_d\dot{\gamma }{d}^2$, where $k_d=0.0186/f$, and this relation is not sensitive to the diameter ratio for $D_{LS}\le 3$. The diffusion coefficient is not sensitive to the friction coefficient except for the extreme condition where $\mu <0.1$, and it is also not sensitive to the restitution coefficient between 0.3 and 0.9.