Effect of modification of poly(ethylene terephthalate) (PET) on the high-speed spinning of PET-SDY or PET-POY was investigated. PET and its copolymers having 3-10 mol% of aromatic isophthalic acid (IPA) or aliphatic diethylene glycol (DEG) were prepared by melt polycondensation to compare the effect of aromatic or aliphatic structure of the comonomers on the crystallization and molecular orientation of the PET fibers. The copolymers having DEG showed lower $T_{g}$ than those having IPA on the basis of same comonomer content though the $T_{m}$ depression of PET by incorporation of the comonomers was similar. It was observed that aromatic IPA was more effective in lowering the rate of crystallization and crystallinity of PET than aliphatic DEG. On the other hand, they showed similar effect in suppression of molecular orientation of the PET fibers. Conclusively, aromatic comonomer seemed to be more effective for the high-speed spinning of PET-SDY, but both of the comonomers were effective for the high-speed spinning of PET-POY.