A Photonic Crystal Fiber (PCF) is a special class of optical fibers which is made of a single material and having air holes in the cladding. This paper studies and compares the optical characteristics such as effective area, confinement loss and nonlinearity, among three different PCF's structures: Hexagonal PCF (HPCF), Octagonal PCF (O-PCF) and Decagonal PCF (D-PCF) with varied structural parameters (number of the air-holes rings, the air-hole's diameter, and the lattice constant), and the target is to use the fiber in a Raman amplifier. Proposed structures are simulated by using COMSOL MULTIPHYSICS which depends on Finite Element Method (FEM). The numerically simulated results shows that Decagonal PCF (D-PCF) offers lower confinement loss, lower effective area, and larger value of nonlinearity than the other two structures. It is seen that Decagonal PCF(D-PCF) is suitable for long transmission fiber applications.