Based on a scalar approximation, the waveguide mode and dispersion property of photonic crystal fibres (PCFs) are investigated with an effective index approach in this paper. It is found that photonic crystal fibres can be single-mode over a very broad spectral range by changing PCFs cladding air-filling fraction or pitch of holes or core diameter. Designing of PCFs can yield zero dispersion at wa velength shorter than 1.27μm and nearly zero flattened dispersion over a very b road spectral range and dispersion compensation with a large normal dispersion.