In this paper the Erbium-Ytterbium (Er3+/Yb3+) co-doped fiber laser pumped by a laser diode is simulated using the Optiwave software. The proposed model is introduced and was used to perform a parametric study on four different samples of Er3+/Yb3+ fiber A, B, C, and D differ in their core diameters and their numerical apertures. The study was done in terms of fiber length, cutoff length, output mirror reflectivity, slope efficiency, and lasing threshold. The simulation results demonstrated that the optimum fiber length is 2 m, 0.5 m, 1.7 m, and 2.3 m for type A, B, C, and D respectively. The max achieved power is 3.15 w for type C at the optimum fiber length. The cut off length is 2.1m, 1.8m, 1.9m, and 2.4m for type A, B, C, and D respectively. Max. optical power of 3.15 w at 40 % reflectivity for type C fiber. Lowest lasing threshold level is 140 mw for type D. The highest slope efficiency is 73% for type C.