The two dimensional with a triangular-lattice cross sectional pattern of elliptic air holes photonic crystal fiber (PCF) is investigated in detail by use of plan wave expansion (PWE) method. Using PWE method, the dependence of dispersion on structure parameters is calculated. We compared the dispersion relations between the circular air holes PCF and the elliptic one. We find that the flatter dispersion curve and single mode in PCF can be realized by adjusting the width and the holes pitch. The single mode can be obtained by rotating the angle of elliptic air hole and two modes will become one mode. As an example, how to design the structure parameters for engineering the chromatic dispersion of elliptic air holes PCF is demonstrated.