Abstract: Fiber lasers technology has grown rapidly due to the rapid advances in high power diodes, diode-to-fiber coupling schemes and doped fiber design and fabrication. Erbium-Ytterbium (Er3+/Yb3+) co-doped fiber is an attractive active medium for the fiber lasers in which Ytterbium is co-doped with Erbium to produce a spectrum in third telecoms window around 1550nm which makes them suitable sources for long range applications. In this paper the Er3+/Yb3+ fiber laser pumped by a laser diode at 980 nm is simulated using the Optiwave software. The pump source was swept from 1.25 to 5 w to extract the slope efficiency. The pump radiation was focused into the Er3+/Yb3+ fiber through an input mirror, which was 98% reflecting at 1550 nm and 99% transmitting at 980 nm. A length of 0.1 m of Er3+/Yb3+ fiber was used with an N.A. of 0.22, Er3+ ion density of 25.4 x 1024 m-3, and Yb3+ ion density of 320 x 1024 m-3. The output mirror was 50% reflecting at 1550 nm. Then the length of the fiber was swept from 1m to 5m in order to obtain optimized fiber length. The simulation results demonstrated that a laser output power of 0.8 W was obtained at 1550 nm for a launched power of 2 W with a slope efficiency of 40% and a lasing threshold of 0.4 W of launched pump power. The results also showed that the optimized fiber length was achieved at 2 m which is in a good agreement with the published similar experimental schemes.