Time dependence of several properties such as mechanical, thermal, and structural properties of partially oriented PTT filaments was investigated. PTT was spun on a extruder and drawn to draw ratios 1.5, 2.0, 2.5 and 3.0, and the time-dependence of the properties of samples with different draw ratios was measured for a month. At lower draw ratio, it was found that the glass transition temperature increases and the endothermic peak by molecular relaxation occurs in the glass transition region with increasing ageing time. Similarly, the birefringence decreases with increasing ageing time at lower draw ratios. But for the PTT POY with a high draw ratio of about 3.0, the mechanical properties, birefringence, and thermal properties did not change with ageing time. It suggests that fiber structure of PTT POY can be stabilized at this draw ratio. Based on these result, it was concluded that the fiber structure of PTT POY could be stabilized if it is drawn lip to about DR 3.0.