The geomorphological feature of the valley's asymmetry is described in the Western Zagros in Iraqi Kurdistan; in terms of facing of the valley sides relative to the position of the sun. The asymmetry is represented by steeper northwest facing valley sides; as compared to the southeast facing sides. This feature shows clear valley's asymmetry in cross section is a new geomorphological characteristic for the Western Zagros Mountain Belt. The asymmetry of valleys, in the present study, is proved to exist in different rock types and areas, which is developed by more chemical weathering of one facing sides relative to the other side. The weathering is attributed to the remaining of the moisture for longer time than the southeastern sides, which are stroke by sun radiation for longer time and are dried more rapidly. A simple method was established for indicating the steeper side of the valleys. The method consists of drawing two parallel lines across the photo of the valley, then connecting the bottom of the valley with the left and right deflection points on the inter-valleys ridges by lines. Finally the angles between the lower horiozontal line and inclined lines are measured, which indicates the asymmetry of the valleys.