The effect of four harvesting systems for thinning tasks (harvester-forwarder, chainsaw-skidder, chainsaw- three-wheeled loader and chainsaw-yarder) was evaluated on percentage of damaged trees and damage surface. Additionally, the effect of the logging system and the season of the operation (winter and summer) on the incidence of damage and surface of the injuries, and their relationship with tree diameter, was also analyzed. Of the four logging systems studied, those carried out with chainsaw-yarder and harvester-forwarder originated the greater incidence of damage with 34.5% and 32.2%, respectively, also producing the greatest injuries with 349.7 and 427.2 cm(2), respectively, by individual tree. The season in which the operations were conducted is a factor that significantly affected on the increase of the number of trees with some sort of damage, with an average incidence of 27.1% in winter and 10.1% in summer. In addition, tree extraction with forwarder and yarder has a high potential for residual stand damage, and the stand structure was not affected by mechanized thinning, because there were damages in all sizes of remaining trees.