Soil improvement is commonly done in construction work with the aim of increasing the bearing capacity of the soil so that it can bear the construction load that will stand on it. The method often used is soil stabilization with the addition of stabilizing agents. In this study, soil stabilization was carried out by adding Portland cement. This research begins with taking soil samples and testing conducted in the laboratory to determine the physical and mechanical properties of the soil, followed by making soil samples for free compressive strength testing and CBR Lab. with 5 variations of cement, from 0%, 5%, 10%, 15% and 20%. From the results of this study, it was found that the soil sample had a moisture content of 30.174%, a specific gravity of 2.413, a liquid limit of 52.46% and a plasticity index of 28.49% with a plastic index value of = 23.97%. Based on the soil classification system using the USCS (Unified Soil Classification System) method, the excavated soil types of the Nipa-nipa Reservoir include the CH (Clay-High) type, clay with high plasticity value. The free compressive strength test on the excavated soil sample without Portland cement obtained the soil compressive strength value of 0.15 kg / cm2. And soil stabilized with various variations of cement obtained the highest compressive strength value of 2.291 kg / cm2, namely the soil sample + Cement 20% (21 days of ripening). For excavated soil the reservoir without Portland Cement has a CBR value of 2,640% and after the addition of Portland cement, it has increased quite significantly, up to 74.948% in the excavated soil sample + 20% Portland cement. The results showed that the excavated soil of the Nipa-nipa reservoir, which was stabilized with Portland cement, can be recommended as a subgrade filling material for Jalan subgrade.