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Gain guided and index antiguided fiber, having a negative refractive index step from the cladding to the core combined with an adequate gain effect, can deliver robust single mode operation with a large mode area.In this paper, we introduce this new concept of gain guided and index antiguided fibers into the common optical device of fiber amplifier, and evaluated the gain characteristics by solving propagation rate equations on the fiber with Yb3+ doped, where the core has a radius of50um, refractive index n0 =1.58, and the index step △n=-0.0045 , concentration of ions 2.43×1026m-3, at the signal (975nm) and the pump (910nm) wavelength and a initial signal power 1μW.According to the function of gain versus fiber length, being obtained from the numerical computation, we can confirm that a properly designed gain guided and index antiguided fiber can provide highly efficient optical amplification with a short length.