The market value of essential oils is steadily increasing every year. They are mainly used as aromas and preservatives in food and cosmetics industries. The content and yield of the obtained extracts change across the seasons of the year. The knowledge of these differences is important for essential oils industry to gain the best quality and quantity of products. In this study Thuja occidentalis L. (Eng. northern white cedar) extract was obtained from leaves by maceration in dichloromethane. Plant material was collected during each season of the year and the finished products were thoroughly examined through GC-MS analysis. The seasonal variations of volatile compound composition showed that the most adequate period to obtain T. occidentalis extract on an industrial scale is spring. This is due to the best efficiency (almost 10%) and the highest content of monoterpenes (67%).