Unboiled and boiled polyester (PET) (Egyptian manufacturer) fibres having irregular cross-sectional shape were stretched at room temperature using a mechanical device with a double-beam interference microscope. The optical results obtained were used for calculating some macro-structural parameters. The evaluated density results were used to calculate the degree of crystallinity, the form birefringence, the mean square density fluctuation and some other macro-structural parameters at different stresses and different boiling times. Relationships between the macro-structural parameters with different stresses are given for these fibres. The investigated parameters changed with boiling and stretching. So the obtained results clarify that reorientation occurred. Empirical formulae were suggested to correlate the changes in some structural parameters with different draw ratios, and their constants were determined. This means that the series and parallel components are deformed in the same mode by drawing.