This research presents a simple index-guiding square photonic crystal fibers (SPCFs) that has a silica core surrounded by air hole with two different diameters. It is demonstrated that the designed two-different-size hole-arrayed index-guiding SPCFs has a ultra-flattened chromatic dispersion of 0 +/- 0.9 ps/(nm center dot km) in a wavelength range of 1.34 to 1.61 mu m and low confinement loss of less than 10(-7) dB/m in a wavelength range of 1.2 to 1.7 mu m. It has also been shown that the proposed SPCFs show reasonable dispersion tolerance.