Research has been conducted on adding a mixture of Polyethylene Terephthalate (PET) and Coconut Fiber Ash (CFA) as a substitute for fine aggregate to see the characteristics of composite concrete. Variations were made to increase the weight percentage of PET by 3%, 5%, and 8% and use CFA by weight percentage of 5%, 10%, and 15% to replace the fine sand aggregate. This study uses a standard concrete mix design by SNI 03-2834-2000, which consists of sand, stone, cement, and water. The test object is cylindrical concrete with dimensions of 20 cm in height and 10 cm in diameter. The tests carried out are concrete density test, compression test, split tensile test, and analysis of the rule of the mixture of composite materials. This test was carried out when the concrete was 28 days old. The test analysis found that the more addition of PET and CFA to the concrete will result in a reduction in compressive strength, split tensile strength, and elastic modulus of concrete. The best value occurred in the percentage of the weight of the mixture of PET 3% and CFA 5% resulting in a concrete compressive strength of 19.48 MPa, a split tensile strength of 6.86 MPa, and an elastic modulus of 19,993 GPa. [ABSTRACT FROM AUTHOR]