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In this paper, we have demonstrated birefringence modification in regenerated grating in polarization maintaining fiber (RGPMF) by using a CO₂ laser annealing technique. After conducting an isothermal annealing procedure followed by a slow cooling process, the birefringence of the RGPMF has been increased. This phenomenon can be explained by the changes in the thermal expansion coefficient and glass transition temperature of the stress applying part at different cooling rates. This process was reversible by reannealing with a subsequent fast cooling process. In our observation, the birefringence exponentially increases with a decreasing cooling rate. The highest and lowest records in the birefringence throughout the annealing process are 4.72 × 10^-4 and 3.46 × 10^-4, respectively (a difference of 1.26 × 10^-4). This finding is useful for the study of the birefringence modification, measurement range, sensitivity, and accuracy of PMF or PMF-related devices.