Prostaglandin (PG) $E_2$, the most abundant prostaglandin in the human body, is synthesized from arachidonic acid via the actions of cyclooxygenase (COX) enzymes. $PGE_2$ exerts homeostatic, cytoprotective, inflammatory, and in some cases anti-inflammatory effects. Also, it has been reported that $PGE_2$ is involved in hair growth. Diphlorethohydroxycarmalol (DPHC) is a phlorotannin compound isolated from the brown algae Ishige okamurae, with various biological activities in vitro and in vivo. In this study, the biological effect and mechanism of action of DPHC on prostaglandin synthesis in HaCaT human keratinocytes was examined. The results showed that, in these cells, DPHC significantly and dose-dependently induced $PGE_2$ synthesis by increasing the protein and mRNA levels of COX-1 and COX-2. Interestingly, DPHC-induced COX-1 expression preceded that of COX-2. Also, while both rofecoxib and indomethacin inhibited $PGE_2$ production, the latter was seems to be the more potent. From above results, we can expect that DPHC has some beneficial effects via increasing of $PGE_2$ production.