The homogenization process caused by competition between phase separation and ester-interchange reactions in immiscible polyester blends was investigated via the Monte Carlo simulation method. Phase separation and ester-interchange reactions were performed simultaneously with the one-site bond fluctuation model on a homogeneous blend of immiscible polyesters. Three different values of the repulsive pair-interaction energy (E-AB) between segments A and B and two trial ratios of phase separation to ester-interchange reactions at a given E-AB were introduced to examine the competition between them. Phase separation was monitored by the calculation of the collective structure factor, and copolymerization was traced by the calculation of the degree of randomness (DR). In all cases, as the homogenization proceeded, the maximum intensity of the collective structure factor initially increased, reached a maximum, and finally decreased, whereas the peak position where the structure factor had a maximum shifted downward in the early stage and then remained unchanged after the intensity of the collective structure factor reached the maximum. This indicates that during the homogenization process, the domain size did not change significantly after phase-separated structures were developed distinctly. In this simulation, phase-separated structures were traced until the DR was above 0.8. This result indicates that homogenization can be accomplished via homogeneous ester-interchange reactions over most of the polyester chains because copolyesters resulting from ester-interchange reactions do not act as an efficient compatibilizer.