In this paper, the dispersion of photonic crystal fibers with square structure is investigated.   First, the effect of changing the diameter of the air holes in each ring around the core is studied separately. In order to minimize the dispersion, particle swarm optimization algorithm is used in which the radii of  air holes in the three rings around the core are considered as a particle. It is demonstrated that the dispersion diagram of the optimized fiber is flat over the wavelength interval of 1.2-1.6 µm and is about 1.8 × 10^-3 ps/(km.nm) at the wavelength of 1.55 µm.