The quality of natural products containing polyphenols, such as Camellia sinensis L. (tea), is highly influenced by oxidation during post-harvest processing and storage. Polyphenol oxidase (PPO) is a key enzyme that accelerates oxidation, which may lead to decreased antioxidant activity and undesirable color changes. This study aimed to evaluate the effects of various thermal and storage treatments on PPO activity, total phenolic content, and antioxidant capacity of tea leaves. Several treatments were applied to investigate differences in the bioactive characteristics of tea leaves after processing. PPO activity was measured using UV-visible spectrophotometry, while total phenolic content and antioxidant activity were analyzed using appropriate methods. The results indicated that one specific thermal treatment effectively reduced PPO activity while helping to maintain higher phenolic content and antioxidant capacity compared to other treatments. These findings support specific processing methods in preserving the bioactive compounds in Camellia sinensis L. leaves.