The Hyrcanian forests are one of the most important global terrestrial ecosystems influenced by environmental factors. They have significant impacts on sustainable development of human communities. The forest monitoring based on phenological trends is essential for understanding the response of forest ecosystems to climate variability. The objective was thus to analyze long-term trends of vegetation activity using Mann Kendall test pixel by pixel on 756 biweekly NOAA NDVI time series in the forest areas of northern Iran during 1981-2012. Trends of temperature and precipitation and their regression relationships with NDVI were considered. The greening and browning trends appeared in 75.35% and 7.41% of these forests areas, respectively. The precipitation trend analysis exhibited no obvious significant trend in most pixels of the study area, however, trend analysis of temperature time series showed significant increasing trends (α = 0.01) in the whole region. Results of regression relationship between time series NDVI and temperature and precipitation demonstrated a weak relationship with changes in vegetation activities and climatological parameters during 1981-2012. The results of this research demonstrated that greening phenomenon in the Hyrcanian forests cannot be solely related to the climatological factors..