A multiple-beam interferometric method is used to study the change of optical orientation function and molecular structure of nylon-6 fibres due to γ-irradiation under vacuum. It was found that γ-radiation causes alignment to the fibre chains in the direction of the fibre axis, this alignment gives an increase in the optical orientation function. A two-beam interferometric method is used to study the changes in optical orientation function on drawn polypropylene fibres. Empirical formulae are suggested to correlate these changes in the optical orientation function with the dose and with the draw ratio. This study aims to show that the multiple-beam and two-beam interferometric methods can be used to study the changes that take place in polymers and fibres by irradiation or drawing.