The stress-strain curves of polyethylene terephthalate monofilaments dwawn in water at 70°C were measured at room temperature, and the effects of molecular orientation and heat-treatment on the yield phenomena, that is, the primary and secondary yielding, are discussed.<br>In the samples with low-orientation, only the primary yielding is observed and accompanied by necking. The tensile dynamic modulus of these samples decreases with increasing strain until the yielding region. The necking is associated with the decrease of yield load with drawing. This decrease of the yield load depends upon not only the number of chains passing through the crosssection of the sample but also on the thermal effects and the state of aggregaton and orientation. Even in the hot drawing the samples fairly shrink after removal of the load, when the chains are highly strained. The primary yield strain of these samples is nearly equal to the shrinkage of the sample.<br>Consequently (in the highly oriented samples) the extension until the primary yield point can be considered as the reverse process to the shrinkage observed after drawing.<br>In general, the fine structures of tde drawing samples are related to the two regions, that is, “region A” and “region B”. The region A corresponds to the amorphous or low lateral order region which has taken part in the shrinking, and the region B is composed of crystallites or high lateral order region termed C and amorphous or low lateral order region A′ which have hardly taken part in the shrinking. In the highly oriented samples, the secondary yielding in the region A and the primary yielding in the region B are observed.<br>In low oriented samples, there is only the region B which is almost the region A′ and only the primary yielding is observed.<br>In the highly oriented samples heat-treated at constant lenghth, the shape of the load-elongation curve near the secondary yield point becomes the same as that of the untreated samples, when 8_??_11% pre-elongation is given. But the changes of the X-ray diffraction intensity and orientation of the (100) and the (010) plane with the pre-elongation until the primary yield region are very small. These facts mean that the cohesive energy density in the region A of the highly oriented samples leveled up by heat-treatmént, decreases with the pre-elongation and becomes the same order as that of the untreated samples.<br>In the highly oriented samples heat-treated in free state, three yield points are observed when some pre-elongation between the secondary and the primary yield point is given.