Using fiber Bragg grating (FBG) as reflectors of the pump light, which could be made by using high power femtosecond pulse laser in the inner cladding of the DCF, shorter fiber is needed in optical fiber amplifiers, while maintaining at least the same performance as that without FBG. Based on the rate and propagation equations, numerical analysis of Er∶ Yb co-doped double cladding fiber amplifiers with and without FBG has been performed. The results show that amplifiers with FBG can reach the same output powers as those of without gratings, but only half-length of fibers are needed, either for forward or backward pumping. With FBG for backward pumping, the highest output power can be acquired for all cases, while using shorter fibers