Based on the semiconductor amplifiing properties and the structure of optical fiber wave guide an InP inner fiber is developed. The InP inner film fiber can be employed as a small size, broadband, and ultra-short fiber amplifier. The quantum size effects of the fiber are emphatically investigated in the work. Using the experimental data, we compare the effective mass approximation (EMA) with effective parameterization within the tight binding (EPTB) models for the accurate description of the quantum size effects in InP. The results show that the EPTB model provides an excellent description of band gap variation over a wide range of sizes. The Bohr diameter and the effective Rydberg energy of InP are calculated. Finally, the amplifiing properties of the InP inner film fiber are discussed due to the quantum size effects. This work is supported by the Nano-technology Special Project of Shanghai. (Grant No. 0352nm019) and National Natural Science Foundation Of China (Grant No. 60477032)