Spica Prunellae has long been used as a significant component in numerous traditional Chinese medicine (TCM) formulas to clinically treat cancers. Previously, Spica Prunellae was shown to promote cancer cell apoptosis and inhibit angiogenesis in vivo and in vitro. To further elucidate the precise mechanism of its tumoricidal activity, the effect of the ethanol extract of Spica Prunellae (EESP) on the proliferation of human colon carcinoma HT-29 cells was elucidated and the underlying molecular mechanisms were investigated. The proliferation of HT-29 cells was evaluated using 3-(4, 5-dimethyl-thiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and colony formation analyses. The cell cycle was determined using fluorescence-activated cell sorting (FACS) with propidium iodide (PI) staining. The mRNA and protein expression of cyclin-dependent kinase 4 (CDK4) and cyclin D1 was examined using RT-PCR and western blotting, respectively. EESP was observed to inhibit HT-29 viability and survival in a dose- and time-dependent manner. Furthermore, EESP treatment blocked G1/S cell cycle progression and reduced the expression of pro-proliferative cyclin D1 and CDK4 at the transcriptional and translational levels. Altogether, these data suggest that the inhibition of cell proliferation via G1/S cell cycle arrest may be one of the mechanisms through which Spica Prunellae treats cancer.