Abstract: The effect of mineralogy on durability of 36 weak rock samples was investigated. Samples were obtained from six different sites with distinct lithologies that included 12 mudstones from two sites, 14 mudshales from two sites, 6 clayshale and 4 siltstones. Slake durability tests were continued to fifth cycle to study the effect of different cycles on durability of weak rocks such as shale. The highest value of slake durability was measure for siltshale and the lowest for clayshlae. The slake durability results of this study also indicate that there is no single parameters that can be used to predict the durability of weak rocks. The results show that durability is closely related to the quantity of clay minerals. Therefore, it can be concluded that variation in durability within weak rocks is controlled predominantly by mineralogy. A decrease in the slake durability was associated with an increase in clay content. It seems that other geological parameter such as fabric and weathering also effected durability of weak rocks.