Structures (road, dam, building etc.) sit on top of soils that should have enough bearing capacity with minimum settlement under structural loads. Bearing capacities of weak soils have been increased with soil improvement methods including grouting methods and geosynthetic materials. In this study, a geocell material was used in order to increase bearing capacity of a medium dense under a strip footing meanwhile effective width of geocell was determined. It is determined that the effective width of geocell under the strip footing is about eight times of the width of strip footing. In addition to that, an increase in bearing capacity of the medium dense sand with geocell reinforcement was three times more than that of the sand without geocell reinforcement.