Poly(ethylene terephthalate) (PET) has been melt-mixed at 293 degrees C with two types of co[poly(ethylene terephthalate-p-oxybenzoate)] (POB-PET) copolyester, designated as P28 and P82, at the level of 10 wt %. The different p-acetoxybenzoic acid/PET mole ratios are 20/80 in P28 and 80/20 in P82. The blends were heated to 305 degrees C and then cooled to 25 degrees C at rate of 10 degrees C/min. The crystalline morphology of these blends in the range from 25 degrees C to 305 degrees C is monitored by polarized light microscopy. Morphologic observations revealed a quicker appearance of crystalline in the blends upon the addition of 10 wt % copolyester as a nucleating agent. It is found that P28, which contains fewer POB units, is much more advantageous to the nucleation process than P82, which contains more POB units. According to the optical photomicrographs, sufficient uniformity of distribution of copolyester domains in the PET matrix is reached after 8 min blending.