Effects of temperature by heating, heating media and draw ratio on the contribution of crystalline and non-crystalline birefringence in drawn and heated, at a constant length, polyethylene terephthalate (P. E. T.) filaments were investigated by X-ray and optical polarized microscope. Results obtained are shown as follows:1) Both density and crystallinity (X-ray) Xx increase with rising temperature, and their values by wet-heat-set are larger than by dry-heat-set under same heated temperature. Relation between Xx (X-ray) and specific volume V calculated from density values may be a linear, independent of draw ratio and heating conditions. Xx=-8.01V+6.002) Relation between Xx (X-ray) and crystallinity Xd deduced from density may be a linear. Effects of draw ratio on the slope of line is small by dry-heat-set, but its slope in lower stretched filaments becomes as much as double higher stretched filaments.3) Both optical birefringence Δn and degree of crystalline orientation π increase with rising temperature. And crystalline orientation factor fx increases with rising temperature, while it may approach to a saturated value at elevated temperature by wet-heat-set.4) Total-birefringence is divided from the contribution of crystalline and non-crystalline birefringences. The contribution of crystalline birefringence Δnc•Xx increases with rising temperature independent of heating condition in drawn filaments. On the other hand the contribution of amorphous birefringence Δna (1-Xx) decreases in higher stretched filaments on dry-heat-set, while it varies little in low stretched.5) The crystalline birefringence Δnc is always larger than the amorphous birefringence Δna. For higher stretched filaments, both Δnc and Δna do not vary much by dry-heat-set, while for lower stretched ones, Δnc, varies a little and Δna increases rapidly with rising temperature.6) Relation between over-all birefringence and amorphous birefringence is a linear in both drawn filaments. And_??_ relative intensity value I(100)/I(110), and total-birefringence are linealy related to each other.