The finite element method is used to investigate the propagation and nonlinearity of octagonal photonic crystal fibers of total internal reflection type.We changed the structural parameters of the fibers and obtained the curves of relations about the propagation and nonlinearity. At last, we found another structure of photonic crystal fiber.It is demonstrated that it is possible to design a low-loss dispersion-flattened PCF at 1.55 micrometer wavelength.