Polyethylene (PE) is widely used as packaging materials in the form of plastic bags. After end-use, some of these plastic bags are not properly disposed and may persist in different kinds of environment. Oxo-biodegradable plastics are PE films incorporated with pro-oxidants to promote degradation. As part of the continuing study, changes on the tensile properties of oxo-biodegradable PE plastic bags immersed in alkaline solution at 60 °C are investigated. The effects of colorant additive (varied at two levels) and thickness (varied at three levels) on tensile properties of PE films are reported. Specifically, the tensile strength [MPa], strain at break [%], and elastic modulus [MPa] are monitored with time. It is observed that films undergo degradation which is exhibited by a decrease in all tensile properties after 1008 hours of immersion in alkaline solution. Analysis of variance (ANOVA) shows that the initial tensile strength and elastic modulus, as well as the strain at break and elastic modulus after 1008 hours of immersion, are all affected by colorant additive.