Cyclic oligomers of poly(ethylene terephthalate)(PET) were separated by HPLC and they were analyzed. Pure cyclic trimer used as the standard for calibration was collected and structural analyses were conducted. A calibration curve for the quantification of the cyclic oligomers was obtained with very high confidence. Also an easy and reproducible sample preparation procedure of dissolution and reprecipitation using co-solvent and condition for high resolution HPLC separation were determined. Total amount of cyclic oligomer in melt polymerized PET with intrinsic viscosity(IV) 0.64 was 2.09$pm$0.08 wt% at 95%-confidence level. The amount of cyclic trimer and tetramer were 1.08$pm$0.06 wt% and 0.31$pm$0.01 wt%, respectively. Also the amount of cyclic oligomer decreased with increasing ring size. Total amount of cyclic oligomer in solid state polymerized PET with IV 0.80 was 0.82$pm$0.04 wt%. The amount of cyclic trimer and tetramer were 0.48$pm$0.03 wt% and 0.11$pm$0.01 wt%, respectively. The amount of cyclic oligomer in solid state polymerized PET was drastically decreased since some oligomers were polymerized and low molecular weight oligomers such as cyclic trimer and tetramer were eliminated during solid state polymerization.