In 1982, Yatagan Thermal Power Plant has started its activity. Some of the forests in the vicinity of the power plant, began to decline in 1983 and in 1985, trees were clear cut in an area of approximately 2271 hectares. It is known that the cause of the damage was due to the effects of the thermal power plant. After the year 1985, the chronic effects could be visualized in a group of trees or in a single tree. In these observations, reducing diameter increment could be seen in trees that were not damaged, yet. Site conditions, stand conditions (density of stand, tree species, and the age of the plant) with the process of silvicultural treatments could be effective on diameter increment of forest trees. In this research, the reducing effect of diameter increment on the forest that are near by Yatagan Thermal Power Plant is studied with surveying the factors that are effective on diameter increment. As a result, it is can be inferred that, since there were no change in site and stand conditions, and the silvicultural treatments, the decline in the diameter increment has been the consequence of air pollution created Yatagan Thermal Power Plant.