A proper characterization of decay heat is essential for nuclear power safety at all stages of the fuel cycle, including reactor operation, spent fuel handling, transportation, reprocessing, and disposal. This work is based on a Monte Carlo approach to quantify the uncertainty propagation of decay data on decay heat calculations using an in-house code named COCODRILO, which is coupled with Serpent2. The calculations are performed for thermal fission pulses of 239 Pu and 235 U using ENDF/B-VII.1 and ENDF-VIII.0 nuclear decay data libraries. Results are also compared with experimental values at different cooling times. The associated analysis shows the impact of decay data uncertainties on decay heat. Moreover, the study underscores the discrepancies in decay heat uncertainty calculation due to decay data without uncertainty information.