High order fibre mode structures are measured by a near-field scanning optical microscope (NSOM) and are determined to be consistent with the standard fibre LP pattes. The mode-field distortions of high-order mode LP11 and LP21 structures by far-field measurement are analysed based on scalar diffraction theory, and the maximum distortions may reach as high as 5.1% and 6.2%. This shows that the method of using the NSOM is more accuracy than the far-field measurement method, especially for high-order mode structures of optical fibres.