Improvement and stabilization of soils are widely used as an alternative to substitute the lacking of suitable material on site. Soils may be stabilized to increase strength and durability or to prevent erosion and dust generation. The use of nontraditional chemical stabilizers in soil improvement is growing daily. A new stabilizing agent was developed to improve the mechanical performance and applicability of clayey soils. In this study a laboratory experiment is conducted to evaluate the effects of plasticity index and waterborne polymer on the Unconfined Compression Strength (UCS) of clayey soils. The laboratory tests include sieve analysis, hydrometer, Atterberg limits, modified compaction and unconfined compression tests. Three clayey soils with different plasticity indexes were mixed with various amounts of polymer (2, 3, 4 and 5%) and compacted at the optimum water content and maximum dry density. The unstabilized and stabilized samples were subjected to unconfined compression tests to determine their strength at different curing times. The results of the tests indicated that the waterborne polymer significantly improved the strength behavior of unsaturated clayey soils. Also, an increase in plasticity index causes a reduction in unconfined compression strength.