The impact of inflation dynamics on the common economic policy in a monetary union was studied using a small New Keynesian model. Two versions of the Phillips curve were considered: one with only forward-looking dynamics and one with both forward and backward-looking dynamics. The findings show that the persistence of inflation in the Phillips curve plays a crucial role in determining the common economic policy in the monetary union. Additionally, when inflation dynamics differ among countries in the union, the country with more persistent inflation has a significant influence on the common economic policy.