6-Hydroxy-2,7-dimethoxy-1,4-phenanthraquinone (PAQ) isolated from the tuberous roots of Yam (Dioscorea batatas) inhibited cyclooxygenase-2 (COX-2) and cyclooxygenase-1 (COX-1) dependent prostaglandin $D_2$ ($PGD_2$) generation in mouse bone marrow-derived mast cells in a concentration-dependent manner with $IC_{50}$ values of $0.08{mu}M$ and $0.27{mu}M$, respectively. In the Western blotting with specific anti-COX-2 antibodies, the decrease of the quantity of $PGD_2$ was accompanied by a decrease in the COX-2 protein level. But PAQ did not affect COX-1 protein level. In addition, this compound inhibited 5-lipoxygenase (5-LOX) dependent production of leukotriene $C_4$ in a dose-dependent manner, with an $IC_{50}$ of $0.032{mu}M$. These results demonstrate that PAQ has a dual COX-2/5-LOX inhibitory activity. This compound also inhibited the degranulation reaction in a dose-dependent manner with an $IC_{50}$ of $2.7{mu}M$. Thus, these results suggest that PAQ may be useful in regulating mast cell-mediated inflammatory diseases.