Transforming growth factor-beta (TGF-beta) signal pathway has an essential role in the induction of catagen phase in hair follicle cycle. 4-O-methylhonokiol, a neolignan compound from Magnolia Officinalis, has various biological activates such as anti-inflammatory, neurite outgrowth activity, anti-acetylcholinesterase activity and hair growth promoting effect. However, the hair-growing mechanisms of 4-O-methylhonokiol on the TGF-beta signal pathway have not yet been elucidated. Thus, we examined the effect of 4-O-methylhonokiol on TGF-beta signal pathway in human keratinocyte HaCaT cells. When HaCaT cells were pretreated with 4-O-methylhonokiol, the TGF-beta1-induced p21 expression was decreased. Moreover, 4-O-methylhonokiol inhibited nuclear translocation of Smad2/3 and Smad4 and Sp1activation by TGF-beta1 . We observed that ERK activation by TGF-beta1 was significantly attenuated by treatment with 4-O-methylhonokiol. On the other hand, TGF-beta has been reported to increase reactive oxygen species (ROS) intracellular content in different cell types as well as the effects of TGF-beta on the cell growth arrest and apoptosis have been kwon to be mediated by oxidative stress. 4-O-methylhonokiol inhibited TGF-beta1-induced ROS production and suppressed mRNA expression of NOX4.These results indicate that 4-O-methylhonokiol could inhibit TGF-beta1-induced cell growth arrest through down regulation of Smad2/3, Smad4, and NOX4 in human keratinocyte HaCaT cell and that the hair-growing activity of 4-O-methylhonokiol might be at least related to its protective action on TGF-beta-induced apoptosis that is assumed to trigger catagen induction in hair cycle.