Acetylbritannilactone (ABL) is a new active compound isolated from Inula Britannica L, a traditional Chinese medicinal herb. It has been reported that ABL can inhibit the proliferation of vascular smooth muscle cells (VSMCs) and neointima formation after balloon injury in rats. ABL also shows chemopreventive properties by inducing cell apoptosis in breast and ovarian cancers, but the antitumor activity and the molecular targets of ABL in colon cancer cells have not been determined. In this study, we showed that ABL inhibits the growth in dose- and time-dependent manners by inducing cell cycle arrest in G0/G1 phase of HT-29 human colon cancer cells. This suppression was accompanied by a strong decrease of cyclin E and CDK4 protein levels, and an increase in p21 protein expression in HT-29 cells. We also show that ABL-induced growth inhibition is associated with the upregulation of KLF4 expression. The overexpression of KLF4 by infection with pAd-KLF4 resulted in growth inhibition, with decrease in the protein levels of cyclin E and CDK4, and increase in the expression of p21, similarly to the effects of ABL. Conversely, knockdown of KLF4 using a specific siRNA impaired the ABL-induced growth inhibition in HT-29 cells. These results suggest that KLF4 as an important cellular target of ABL mediates the growth inhibition of HT-29 cells induced by ABL via upregulation of p21 expression.