The effective resonance temperature (Teff) correlation of UO2 pellets based on experimental power coefficient data from PWR cores, namely, YANKEE, SAXTON, BR-3, and SELNI, is presented. The correlation can predict the total power defect of reactivity from hot zero power to full power within experimental uncertainties and Teff at any power level within ± 80°F (± 45°C), a magnitude which constitutes no more than ± 6% of the value at full power for current PWR design. With the ± 80°F uncertainty in Teff, the magnitude of the power coefficient of reactivity can vary as much as ± 30% in the low power region and ± 20% in the high power region. This study indicates the temperature drop in the gap between the pellet and clad is much lower than the value calculated for it in the past.