A series of 5-arylidene-2,4-thiazolidinediones and its geranyloxy or prenyloxy derivative were synthesized and studied for their radical scavenging activity using 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay. Their comparable scavenging activities were expressed as IC50 value. Compounds 2c, 2d, 4d, and 6a showed appreciable radical scavenging activities. The vanillin based thiazolidinedione compound 2c displayed highest activity comparable to that of alpha-tocopherol. But in vivo, compound 6a showed better results in inducing phase II detoxifying/antioxidative enzyme.