Caffeic acid and its four polymers isolated from Salviae Miltiorrhizae Radix were examined for their activity of scavenging free radicals in a 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical generating system. The results showed that the polymers of caffeic acid inhibited the DPPH radical more strongly than caffeic acid. The strongest activity was displayed by two tetramers, lithospermic acid B and its Mg2+ salt. The trimer (lithospermic acid) and dimer (rosmarinic acid) showed similar efficiency. In comparison, caffeic acid was less efficient in scavenging free radicals. Determination of the activity of caffeic acid derivatives of small molecules revealed that the o-dihydroxyl group was the most important active structure of caffeic acid derivatives for scavenging of free radicals. Lack or substitution of this structure resulted in marked reduction or even loss of the activity. Structural modification of the side chain of caffeic acid produced slight changes in activity. The present results demonstrate that a saturated group connected to the aromatic ring has slightly higher inhibitory activity against the DPPH radical than an unsaturated group.