This study evaluates the influence of recycled PET particle size on enhancing sandy subgrades for flexible pavements. A soil classified as poorly graded sand with silt and gravel (SP-SM) was treated with crushed PET in proportions of 2%, 5%, and 7% to examine its impact on load-bearing capacity. Modified Proctor and CBR tests were conducted to analyze changes in the soil's mechanical properties, revealing that adding 5% PET increased the CBR Index to 15.2% at 95% MDS, an 83.1% improvement compared to natural soil. However, with a 7% PET addition, the CBR was 9.6% at 95% MDS and 15.8% at 100% MDS, showing a decrease in load-bearing capacity likely due to excessive particle interference within the soil matrix. These findings highlight that both PET dosage and particle size are critical for optimizing subgrade performance. This research provides an innovative and sustainable solution for re...