The anti-inflammatory activities of alpha-truxillic acid (1) and 4,4'-dihydroxy-alpha-truxillic acid (2) as well as their monomer components (E)-cinnamic acid (3) and (E)-p-coumaric acid (4) were evaluated in the formalin test. alpha-Truxillic acid (1) and its derivative 4,4'-dihydroxy-alpha-truxillic acid (2) exhibited significant activity against inflammatory pain response, while their monomer components (E)-cinnamic acid (3) and (E)-p-coumaric acid (4) did not show any activity against either neurogenic or inflammatory pain responses induced by formalin in mice. These results suggested that the dimeric structure might play an important role for the expression of anti-inflammatory activity. Furthermore, in order to gain information on their potencies, their anti-inflammatory activities were compared with that of incarvillateine (5) which contains the same dimeric structure and showed more potent antinociceptive activity than morphine in the formalin test. The activities of alpha-truxillic acid (1) and 4,4'-dihydroxy-alpha-truxillic acid (2) at the dose of 40 mg/kg against inflammatory pain response were equal to that of incarvillateine at doses of 20 mg/kg.