Plants have evolved both physical and chemical defenses to make the nutrients of attacked organs difficult to access or more toxic to resist animal consumption or/and pathogen attack. Although it is intuitive that a tradeoff could exist between physical and chemical defenses because of finite defense resources, many studies have failed to detect this tradeoff. We hypothesized that tradeoff between physical and chemical defenses in individual organs was mediated by the total resource allocation to those organs. In this study, we tested whether a tradeoff between physical (i.e. fiber content, which has proved to be a good indicator of investment into seed coat) and chemical defenses (i.e. total phenolics, which are abundant chemical defenses in plant seeds) existed in plant seeds by using 163 common species collected from Xishuangbanna tropical forest, southwest China. Then we tested whether this tradeoff was mediated by seed mass which could be a potential proxy of total resource investment per seed. Among the 163 species, there was large interspecific variation in both total phenolics (from 0.01 to 20.52%) and fiber content (from 4.47 to 81.49%). Our results supported our hypothesis: negative relationships between physical and chemical defenses were much stronger among small seeds than among large seeds. Our study suggests that total resource acquisition must be considered when evaluating defense tradeoffs. However, it is usually extremely difficult to measure this resource acquisition variation, thus we suggest utilizing easily measured proxies of acquisition variation to quantify tradeoffs.