15,16-Dihydrotanshinone I (DHTS) is a component of the traditional Chinese medicinal plant Salvia miltiorrhiza Bunge. In this study, DHTS at as low as 2.5 μg/ml concentration significantly inhibited proliferation of human benign (SW480) and malignant (SW620) colorectal cancer cells, as shown by 3-(4,5)-dimethylthiahiazo (-z-y1)-3,5-diphenytetrazoliumromide (MTT) and flow cytometric analysis. Activating transcription factor (ATF)-3, a basic leucine zipper-type transcription factor, was found to be predominantly up-regulated in DHTS-treated SW480 and SW620 cells. The up-regulation of ATF3 was blocked by a c-JUN N-terminal kinase (JNK) or p38 inhibitor. Overexpression of ATF3 resulted in a significant augmentation of DHTS-induced apoptosis of SW480 cells, but resistance to DHTS-induced apoptosis of SW620 cells. These results suggest that DHTS has a strong therapeutic or preventive potential against cancer. In addition, ATF3 has a dual role in DHTS-induced apoptosis, depending on the degree of malignancy of colorectal cancer.