The samples of Er3+ and Eu3+ co-doped GaN epitaxial films were prepared by ion implantation technique. Their optical properties and energy transfer mechanism were investigated using cathodoluminescence. For Er3+ and Eu3+ co-doped GaN films sample, the mixed color of green and red light can be realized at 300 K. Moreover, the emission intensity of Eu3+ ions and intensity ratio of Er3+ ions were decreased with the increase of Er3+ ions implantation dose, which proved the existence of the energy transfer from Eu3+ to Er3+ in the GaN host. The result of temperature-dependent cathodoluminescence spectra shows that the intensity ratio is decreased with the temperature increasing, which indicates there is a thermal coupling of 2H11/2 and 4S3/2 states of Er3+ ions. The calculated chromaticity coordinates and color correlated temperature can be tuned through adjusting the implantation dose ratio of Er3+ to Eu3+. This work shows Er3+ and Eu3+ co-doped GaN films are promising materials applied in optical devices.