The thermostability of antioxidant activity of dieckol, a phlorotannin isolated from brown seaweed Ecklonia cava was investigated. The thermostable antioxidant properties of dieckol were evaluated at 30, 60, and $90^{circ}C$ for 7 days using 1,1-diphenyl-2-picrylhydrazyl (DPPH) and hydroxyl radical scavenging activities, and comparing its performance to that of ascorbic acid. The intracellular reactive oxygen species (ROS) scavenging activity and apoptotic body formation were investigated using DCF-DA assay and nuclear staining with Hoechst 33342, propidium iodide and flow cytometry. Dieckol treated at different temperatures during 7 days showed stable scavenging activities on towards DPPH and hydroxyl radicals. In addition, dieckol showed a stable protective effect against $H_2O_2$-induced apoptotic body formation in Vero cells. On the other hand, the radical scavenging activities and intracellular ROS scavenging activities of ascorbic acid, used as a positive control, were significantly decreased at $60^{circ}C$ and $90^{circ}C$ from on the 4th day and 3rd days, respectively. In conclusion, the results indicated that food grade antioxidant extracts containing dieckol derived from E. cava remain a stable during the temperatures encountered during the processing of food and cosmetics.