One of the main soft rock characteristics are relatively rapid deterioration and degradation of strength when soft rock is exposed to atmospheric agents. In the investigation of strength degradation caused by weathering, wetting-drying process (as a part of weathering) is the factor that has most influence. Deterioration of shear strength due to weathering simulated by wetting-drying is tested on marl samples from Eocene flysch formation. Measurement is carried out with a portable shear apparatus, and a modified process of the sample preparation is implemented. Weathering of samples was simulated in laboratory conditions. Groups of five samples were subjected to 2 to 8 cycles of this process. The results provided the conclusion that the magnitude of the strength degradation depends on the strength of unweathered material and the number of simulated weathering cycles. However, with the increase of wetting-drying cycles, the changes become insignificant.