The effects of ester interchange reactions on the phase behavior of an immiscible polyester blend have been investigated using the Monte Carlo simulation method. Two kinds of ester interchange reactions such as alcoholysis (or acidolysis) and direct ester-ester interchange are simulated by end attack and bond flip, respectively. The chain movement is performed by the one-site bond fluctuation model. First, the phase-separated structure is prepared by introducing the pair interaction energy E-AB between segments A and B. As the ester interchange reactions at the interface between the two phases proceed, the degree of randomness increases and the maximum intensity of the collective structure factor decreases, whereas the peak position at which the structure factor has a maximum remains almost unchanged. These results indicate that the homogenization of the phase-separated structure of the immiscible polyester blend proceeds by the ester interchange reaction without a change in the domain size, while the concentration difference between the two phases becomes smaller. These results are also confirmed by analyzing the changes in concentration profiles.