Stress is a kind of physical or psychological exhaustion caused by actual or superficial problems. When pressures in working environment build up, a considerable impact on body and mind can be developed. This condition may lead the worker in to a harmful accident and bring about a dangerous situation. In many countries forestry is one of the toughest and most difficult types of occupation. Working in forest is intensive and involves a high degree of risk. Operating chainsaw and skidder are known as two most stressful occupations in forest. In order to study effect of occupational stress on mental health of forest workers, a research was conducted on the workers of two companies Mazandaran Wood and Paper Manufactory and Nekachoob Wood Technology using GHQ-28 questionnaire (General Health Questionnaire). Furthermore, in order to investigate and compare forest workers under impact with greater details, control group was also considered. Kroskal-Wallis test showed that there was a significant difference between target and contro groups in terms of mental health at 99 percent statistical levels. Working conditions and occupational stress had negative impact on mental health of both chainsaw and skidder operators so seriously that they showed symptoms like anxiety, nervousness, lack of sleep, etc.