The changes of mechanical properties of poly(ethylene terephthalate) filament were studied in relation to crystallinity and structural factors, when it was drawn above Tg, 90$^{circ}C$, and below Tg, 25$^{circ}C$, at various draw ratio and it was heat set at 100$^{circ}C$, 150$^{circ}C$, 200$^{circ}C$, While the yarn was 1)free to relax (FA), and 2) held taut at constant length (TA). The drawn samples at 25$^{circ}C$ have higher strength, initial modulus and birefringence which indicated the overall orientation of drawn PET filament and have lower strain and crystallinity than those at 90$^{circ}C$. And all of mechanical properties except breaking strain improved with increasing draw ratio. After heat-setting, the mechanical properties and structural factors were also shown similar behavior. For the effects of heat-setting temperature, the breaking strength and initial modulus decreased with increase in heat-setting temperature but the degree of crystallinity increased. The breaking strain of FA samples had no change by the heat-setting temperature because of the difference of heat-setting conditions. For the dependence of birefringence on the heat-setting temperature for free and taut annealed samples, the decrease of all the free annealed samples and the increase for taut annealed samples were expected results as also observed by other authors. On the base of abovementioned experimental results, the mechanical properties of drawn samples were affected by the overall molecular chain orientation rather than by the crystallinity of a polymer. And it was found that the samples drawn at high temperature had greater crystallinity than those at low temperature, even the samples with the same draw ratio.