The apparatus employed to measure the temperature dependence of the birefringence on synthetic fiber was improved. The temperature dependence of the birefringence and static tension on P. E. T. filament, drawn and heated at a constant length, varying draw ratio and heating condition, were measured over a wide range of temperature under a constant heating and cooling rates.<br>The optical-stress coefficient (Δn/f) for successive heating and cooling at the various temperature was investigated. The results are shown as follows:<br>1) The temperature dependence of the optical-stress coefficient on the un-treated P. E. T. filaments is that Δn/f decreases rapidly up to the transition temperature, independent of draw ratio, and begins to increase near the region, whereas beyond this region, it increases linearly with the increase of temperature. The rate of the increase for a lower stretched filament is larger than for a higher one. The value of Δn/f at various extensions on un-treated P. E. T. filament increases with rising temperature and changes in the vicinity of transformation temperature on the statictension (f) vs. temperature curve of P. E. T. filament. This transformation temperature may be moved towards higher temperature with the increase of extension.<br>2) Relation of the temperature dependence on the optical-stress coefficient at a constant length for dry-heated P. E. T. filament is quite similiar to the behavior on the un-treated filament. The curvature of the (Δn/f) vs. (T) curves near the transformation temperature becomes larger and shifts towards higher temperature with rise of temperature by heating. And, the transformation temperature of (Δn/f) vs. (T) curves for higher stretched filament is always higher than for lower one.<br>3) The rate of decrease of Δn/f within the lower temperature in Δn/f_??_T curves of P. E. T. filaments, which were heated in various media of heating, decrease and the transformation temperatures shift towards higher temperature in the order of heat treatment by dry air, steam and hot water.<br>4) The temperature dependence of the heated P. E. T. filament on the birefringence (Δn) at a constant length is that Δn exhibits a minimum value near transformation temperature, and beyond its temperature, Δn increases with rising temperature. Its rate of increase is smaller for the temperature raised by heating, but the value of Δn in the (Δn) vs. (T) relation of the heated filament at the most elevated temperature (210°C) becomes lower within the higher temperature than its value at just before thr rise of temperature. The variation of Δn in (Δn)_??_(T) curves of the dry-heated higher stretched P. E. T. filament is smaller as compared with that of the lower stretched.