Short-Abstract-We investigated the Zn concentration dependence of Zn diffusion from Zn-doped InP (Zn-InP) to Rudoped InP (Ru-InP). As semi-insulated InP, Ru-InP shows advantages in terms of higher resistivity and lower capacitance when Ru-InP is sandwiched by Zn-InP. Control of the diffusion of Zn from the adjacent Zn-InP layer is required to achieve laser diodes with superior characteristics. In this paper, we examined the diffusion of Zn at a Ru-InP stacking structure sandwiched by Zn-InP using metal-organic vapor phase epitaxy.