Abstarct: Our country, Iran due to special geological, topography and climate conditions are suffered significant economic lost due to occurrence of landslides annually. In recent decades, the growth of population and requirements to food supply and water management lead to increase constructing of new engineering projects, so we are seeing increasing intense the rate of landslides and its related damages. The Vanyar reservoir dam is located in 5 kilometers northeast of Tabriz in the downward of Vanyar village over the Ajechi river. The dam type is earth dam with an impervious clay core and the height and crest width of dam are 83 and 10 meters, respectively. The area and volume of reservoir are km2 34.7 and km3 361.2. The construction and water reserving in the vicinity of unstable or slopes with potential of instability will be affecting the stability of hills and results to new slide. In this research, the instability of slopes in margin of Vaniyar reservoir after the dam operation and their environmental effects are investigated. For this, engineering geology map and slope classification map are prepared. Landslide inventory map and the characteristics of rock outcrops and alluvial in reservoir area were provided baxsed on the field survey and exploration boreholes. Then, the safety factors of 750 of slopes in margin and over the reservoir were estimated by the Slide 5.0 software in condition of dam operation and design earthquake and finally the hazard zoning map of reservoir provided. The results shown that about 0.5% of slops are classified in unstable category, about 19.5% in partially unstable and 80% in stable group. The occurrence of landslides in dam reservoir will have negative environmental effects such as soil and plant destruction, increase rate of erosion, turbulence of water, decrease of reservoir water quality, increase of reservoir sediment and finally impair to dam and its accessory. In this study, about 18 old slides landslides have been recognized that 4 of them are deep and reactivation of these slides could be results to 1407000m3 sediments in reservoir or which is 0.4% of reservoir volume. Unstability and slides of all unstable and partially unstable slopes and old slides can inter about 14280100m3 sediments into dam reservoir which is relatively high volume.