Forest Ecosystem in addition to timber production has many social and environmental benefits. One of the most important benefits from the forest ecosystem is their role in air pollutants absorption and reducing the pollution level. In this study the economic value of SO2 absorption by forest has been estimated in a part of Caspian forest. At the first, the amount of absorbed SO2 has been calculated by existing models, separately for forest and rangeland that are two main land uses in the region. Then this function has been valuated with replacement cost method. The results show a value of 2475 thousand Rials for absorption of each ton of SO2. According to the area of region that is 20582 hectare, the total amount of SO2 absorption is 42.723 ton per year and its value is 106 million Rials per year for the ecosystem function of SO2 absorption.