Poly(ethylene terephthalate) (PET) is a semi crystalline alipharomatic polyester possing interesting physical and mechanical properties. The correlation between the fiber structure and mechanical properties of two different poly(ethylene terphthalate) fibers types, that is anti pill(wool) and ordinary(cotton) types, was studied. Fiber structure was determined using different analytical methods. The effects of drawing and heat setting conditions on structure of PET fibers were investigated by using birefringence, crystallinity measurments. The crystallization behaviour of PET fibers under relaxed conditions was investigated by using Wide Angle X-Ray (WAXD),Differentional Scanning Calorimetery (DSC), Fourier Transform InfraRed (FTIR) and density measurments. Significant difference in the microstructure of both types was observed. A better developed crystalline structure composed of bigger, more perfect and more axially oriented crystallites was characterized for the anti pill types of PET types. Generally, crystallinity for cotton type was higher in comparison with the wool type of fibers. In addition, amorphous and average molecular orientation is higher. The better mechanical properties of conventional PET fiber type, as demonstrated by a higer higher breaking tenacity and modulus accompanied by lower breaking elongational, are due to the observed structural characteristics.