This study investigates the potential use of waste tire and polyethylene terephthalate (PET) plastic as sustainable stabilization additives to improve the California Bearing Ratio (CBR) value of silt soil. Silt soil, known for their low bearing capacity and poor compaction properties, poses significant challenges for infrastructure development in rapidly growing areas such as Pantai Indah Kapuk (PIK) 2, Tangerang. In this study, the soil was mixed with 2, 4, 6, and 8% waste tire and 0.2, 1, 2, and 4% PET plastic by dry weight of soil, using materials passing through a 4.75 mm sieve. Standard Proctor and CBR tests were conducted to compare the natural soil and treated samples. Natural silt soil exhibited a CBR value of 11.9%, while the mixture of waste tire material with 4% proportion recorded a CBR value of 16%, indicating an increase of approximately of 33.6% and for the mixture of PET plastic material with 2% addition yielded a CBR value of 14.7%, which equals an increase of about 22.7% compared to the natural soil. These findings highlight the potential of using waste tire and PET plastic as alternative stabilizers, offering a technically effective, cost-saving method and environmentally sustainable solution for improving the subgrade soil performance in geotechnical engineering uses.