Nine known diarylheptanoids (1-9) isolated from the barks of Alnus japonica were evaluated for their inhibitory activities on nitric oxide (NO) and prostagrandin $E_2$ (COX-2) production in interferon-${gamma}$ (INF-${gamma}$) and lipopolysaccharide (LPS)-activated RAW 264.7 cells in vitro. The NO and COX-2 levels were moderately reduced by the addition of compounds (1-9). Among these compounds, compounds 6 and 8 inhibited NO production in a dose dependent manner with an $IC_{50}$ of 16.7 and $27.2;{mu}g/mL$, respectively (positive control, L-NMMA; $22.8;{mu}g/mL$), and compounds 6, 7, 8, and 9 reduced the COX-2 level in a dose dependent manner with an $IC_{50}$ of 20.7, 25.7, 25.0, and $27.3;{mu}g/mL$, respectively (positive control, indomethacin; $26.2;{mu}g/mL$). An analysis of the structural activity relationship among these diarylheptanoids suggests that the presence of a keto-enol group in the heptane moiety or a caffeoyl group in the aromatic ring were important for the efficacy on the inhibitory activities of NO and COX-2 production.