In the previous papers, the drawing mechanisms of amorphous PET fibres were discussed. In this paper, undrawn PET fibres are crystallized in water over the temperature range 80_??_100°C, and drawn in water at 70°C and 75°C. Then, the drawing mechanisms are discussed from the measurements of birefringence, density and orientation of crystallites. The results obtained are as follows:1. The initial slope of birefringence-draw ratio curves and the maximum value of the birefringence increase with increasing crystallinity in the process of primary crystallization.2. Though the crystallinity after completion of primary crystallization is about 20%, birefringence-draw ratio curves are no more affected by the change of crystallinity during the secondary crystallization, and it becomes independent of drawing temperature. Drawing behaviours of Nylon 6 are same as PET fibres crystallized to this degree.3. In the hot drawing of samples crystallized to the middle process of primary crystallization in water above 90°C, an irregular structure appears under a polarizing microscope.4. The birefringence-draw ratio curves for samples crystallized to completion of primary crystallization may be represented by following equations;where v1 and Δn1 are respectively the volume fraction and birefringence of the part which is deformed in accordance with Kratky's “first borderline case;” v2 and Δn2 are respectively the volume fraction and birefringence of the part which is deformed in accordance with Kuhn and Grun's theory for rubber photo-elasticity; α the draw ratio; the mean square of direction cosine of orientation units for fibre axis; Δn0 and A the constant.5. It may be considered that vl and v2 are 0.2_??_0.3 and 0.7_??_0.8 respectively until draw ratio 3.6; but v1 increases gradually with increasing draw ratio above 3.6 and becomes one at draw ratio 5.5 and Kuhn and Grun's theory can not be applied for this process.6. It may be considered that the part deformed in accordance with Kratky's theory is composed of crystallites, and the other part deformed with Kuhn and Grun's theory is composed of amorphous molecular chains and a part of crystallites unfolded by drawing.