A chemically stable Er3+/Yb3+ codoped phosphate glass is prepared and used in fabrication of planar waveguides for optical amplifiers and lasers. The weight loss rate of this phophate glass is 4.7×10-5g·cm-2·hr-1, which is smaller than that of Kiger′s commercial phosphate glass QX/Er. Planar optical waveguides are formed by Ag+-Li+ ion exchange and the effective refractive indices are determined by m-lines at 632.8nm. Diffusion parameters such as ion exchange depth, surface refractive indices, variation of refractive index and diffusion coefficient are calculated based on refractive index profiles obtained by Inverse WKB method.