Natural River sand is a term used for aggregates having size less than 4.75mm obtained from natural disintegration of rock and deposited in riverbeds. It is mainly excavated from river beds and most of the time contains high percentages of silt and clay and organic materials that adversely affect the strength of concrete .The cleanliness of the aggregates is required by all standards of construction. Among the undesirable matters in the aggregates, there is silt and clay. The main objective of this research is to investigate the quality of natural river sand in and around Mekelle and investigate the effect of river sand quality on the workability and compressive strength of concrete. For this study, eight (8) natural river sand samples were collected and their physical properties were studied. The result of the investigation showed that the level of silt and clay content ranged from 15.22% to 4.28% for unwashed river sand. Only one sand source out of the 8 meets this limit, representing only 12.5%. An overwhelming majority (87.5%) of the tested sand samples failed to meet silt and clay content limits set in the Ethiopian Standard. For one river sand sample, experiments were performed on a C-25 concrete grades mix with 0.491 W/C ratios with a slump of 25-50mm. Concrete Cubes were casted from the selected one river sand sample batch of the mix. In this research, the specimens were tested at the age of 3, 7, 14 and 28 days. The silt and clay quantities in fine aggregate which have been considered in this study are 1%, 3%, 5%, 7% 9% ,and 12% whereas, all other parameters are kept constant, all results were compared to the properties of concrete which does not contain silt and clay particles (control mix,0%). It was found that the workability and the compressive strength of concrete is inversely proportional to the quantity of silt and clay particles contained in the fine aggregates. A total of 9 concrete cubes (42.85%) made from river sand with varying sand , silt and clay content failed to meet the design strength of 25 MPa at the age of 28 days.