Index-guiding triangular PCF with air core is proposed which guides light by total internal reflection (TIR) when the air core is smaller than the air holes in cladding. Dispersion property is investigated using full-vectorial plane-wave expansion (PWE) method. The impacts of the air core on the dispersion property are demonstrated and PCF with dispersion values between -10±0.5 ps/（nm·km） from 1360 nm to 1730 nm and dispersion slope between ±0.01 ps/（nm2·km） from 1370 nm to 1740 nm is obtained.