Artificial fine aggregate synthesized from waste HDPE plastic was incorporated into the concrete mix at 2.5%, 5%, 7.5%, 10%, and 12.5% by weight of the natural fine aggregate. Various tests were performed on both freshly mixed and hardened concrete. Concrete samples, including 150 mm cubes and 100x100x500 mm beams, were cast to evaluate mechanical properties such as compressive strength and flexural strength. The test results indicated that the mix with a 10% replacement of artificial fine aggregate (AFA) improved compressive strength by 6.8% and flexural strength by 4.9% compared to those using natural aggregates. This demonstrates that using plastic-based artificial aggregates enhances concrete properties and supports sustainability.