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The modal spot size of the fundamental modes in axially nonsymmetrical fibres with axially nonsymmetrical cross-section is theoretically investigated. As the definition of modal spot size, two methods are proposed based on the quadratic moment of light power and the Gaussian fitting of the field. Simulated results using the finite-element method are presented for typical stress-applied polarisation-main taming fibres. It is shown that the spot shape depends mark edly both on the position of stress-applying parts and on normalised frequency.