In this study, to investigate the effect of capillary diameter of spinneret on fiber structure in melt spinning process, melt spinning of PET of I.V. 1.21 dl/g was conducted by using spinnerets with different capillary diameter. Characteristic relationship between the thermo-mechanical properties of obtained as-spun PET fibers and the spinning conditions such as shear rate and draft influenced by capillary diameter was studied focusing on the control of melt structure of polymer in detail. As expected, fiber orientation and structure development increased with draft influenced by capillary diameter and spinning velocity. However, in the case of as-spun fibers obtained at low spinning velocity; low draft, toughness showed a tendency to increase as capillary diameter decrease. These results indicate that melt structure of polymer can be controlled by the capillary conditions.