A new simple structure of an index-guiding highly nonlinear dispersion-flattened square photonic crystal fiber (HNDFSPCF) with low confinement losses is proposed. The results reveal that it is possible to design five-rings HNDF-SPCFs with a flattened dispersion of 0.43 ps/(nm km), low dispersion slope of -0.02 ps/(nm(2) . km), low confinement loss of approximately 10(-3) dB/m, and a large nonlinear coefficient of approximately 35 W(-1) km(-1) at 1.55 mu m. It is also observed that the confinement loss is less than 10(-1) dB/m in the wavelength range of 1.2-1.7 mu m. (c) 2007 The Optical Society of Japan.