Many factors like topography, climate, geology, tectonic, vegetation, soil properties,… are effective for

occurring mass movements, that some of them are natural and some are created due to human activities.

Deforestation and degradation of forests is one of the factors that aggravate the landslide occurrence and

create mass movements directly. To investigate this important problem, in this research, two hillslopes with

the same conditions (from view points of geology, pedology, topography and climate) in the field (Ardal

township- Chahar Mahal & Bakhtiari province) were selected as one of them includes deforestation (unstable)

and another one has been covered by oak forest and is stable. Based on the studies, both hillslopes

have been located in alluvial formation with marl layers and soil texture in both of them is also clay-loam

that dominant mineral clay is esmectite. From point of view of soil mechanical properties (internal friction

angle and soil cohesion), both hillslopes are the same and they classified in ML-CL groups. From the

hydrology view point, water resources in upstream of both hillslopes are common and the rate of infiltrated

water is 152600 cubic meters per year. The most difference between these two hillslopes is the vegetation

percent as one of them has been covered by oak forest with 40 percent canopy while other one (unstable

hillslope) has less than 5 percent canopy. After determining the all effective parameters in landslide, it

was specified that in landslidy hillslope, the factor of safety is equal 1 (Bishop method) and hillslope has

been slided. On the other hand, in hillslope with oak trees, the rate of safety factor (without considering

the effect of tree roots) has been obtained equal to 0.9 and hillslope is stable yet. Then, it can be concluded

that oak trees with 3-4 root depth, have increased the soil strength and its consolidation. Furthermore,

oak forest has been able to extract the further water and decrease the mass weight through increasing the

evapotranspiration. Finally, it can be concluded that in Zagros region, deforestation and cutting the oaks is

one of the important factors for occurring landslides.