This study aims to determine the effect of gamma irradiation doses and time of storage on the mechanical properties of HDPE packaging plastics, and to apply gamma irradiation techniques to improve the mechanical quality of materials, especially plastic-based products. Packaging plastic is the most popular packaging material used, many businesses choose plastic as packaging for their products Research Place for Polymer laboratories, Isotopes and Radiation Technology Application Centers, National Nuclear Energy Agency, Pasar Jumat, South Jakarta and Mechanical Test Laboratory of the faculty of Engineering, UHAMKA Pasar Rebo, East Jakarta. To determine the resistance of packaging plastic to gamma irradiation, the effect of gamma irradiation dosage and storage time on the mechanical properties of HDPE plastic packaging were carried out. The research will be conducted with Cobalt-60 gamma radiation sources at irradiation doses of 10, 15, 20, 25, and 30 kGy with a dose rate of 6 kGy / hour, then some irradiated plastic footage is stored at room temperature for 0 and 5 weeks. The mechanical properties tested were yield strength, tensile strength, and elongation at break using the strograph R1 tensile test tool. Gamma irradiation caused yield strength, tensile strength and elongation in the A brand HDPE packaging plastic increased from 0 to 5 kGy, then decreased by 5.31% at a dose of 10 kGy and then continued to increase to a dose of 25 kGy. The optimum increase occurred from doses 15 to 20 kGy, as much as 12.00%. For plastic B, the breakout extension (Eb) continues to increase as the irradiation dose increases to 30 kGy. The optimum Eb increase occurs from a dose of 0 to 5 kGy, at 9.80%. Increased mechanical properties are caused by free radical species formed during the irradiation process, so that a cross-bond is formed between HDPE plastic packaging molecules. While the decrease occurs due to the weakening of the cross bond between the HDPE packaging plastic molecules.