Poly (ethylene terephthalate) (PET) fibers containing poly (oxyethylene) (PEO) were prepared to study the molecular weight effect of PEO upon antistatic property. A series of fibers containing 2wt% of PEO with molecular weight Mn=4, 000 (PEO (4)), 20, 000 (PEO (20)), and 100, 000 (PEO (100)) as well as the fiber containing 10wt% of PEO (20) were spun by a melt spinning at 270°-280°C, and then submitted to 4.9-fold drawing and heat-treated at 170°C. Each fiber was washed in a boiling water for 1hr. Observation of the transversal sections of the unwashed fibers by light (LM) and scanning electron microscopes (SEM) showed that PEO particles are distributed in the PET matrix. The particle size of PEO in the as-spun fibers increased markedly with increasing the Mn, and its size and number decreased after drawing. The antistatic property of the drawn fibers evaluated by half-lifetime of leakage of static charge was improved with the increase in PEO's Mn. Therefore, this tendency is to be explained in connection with its particle size. The particle size in the sample containing 10wt% PEO (20) was the same as that in the sample containing 2wt% PEO (20), though the number was much larger than that of the latter sample. For the both fibers, the antistatic properties before washing were in the same level. The LM and SEM photographs of the washed fibers indicated that PEO particles bled out from the interior of fiber, with the fibers' inner structure being porous. This observation is correlated with the deterioration of antistatic property after washing. The rate of bleed-out of PEO (20), however, was somewhat slower than that of other PEOs, i.e., the PEO (20) particles still remained on the fiber surface after washing. This may explain the fact that the antistatic property of the fiber containing PEO (20) was unchanged or improved after washing. The viscoelastic nature of those blend fibers was also examined. It showed that the compatibility of PEO with PET decreased with the molecular weight increase of PEO. A tendency to deterioration of mechanical properties of the blend fiber is consistent with this incompatibility. It can be concluded from these findings that blending about 2wt% PEO (20) is the most effective as far as homopolymer of PEO is used for antistatic modification of polyester.