Undaria pinnatifida (UP) has long been ingested by woman who was delivered of a child. The present study was investigated to find out the contractive compound from UP and the contractive mechanism. The active extract, MeOH extract was partitioned with $CHCl_3$ and water. Whereas potassium iodide and alginic acid do not affected the uterus contraction, $CHCl_3$ extract significantly increased uterus contraction in vitro and in situ studies. The contraction by $CHCl_3$ extract was not blocked by atropine, prazosin and propranolol. Blood estrogen level was not changed by $CHCl_3$ extract.