Inhibitor of DNA binding 1 (Id1) plays an important role in genesis and metastatic progression of prostatecancer. We previously reported that down regulation of Id1 by small interfering RNA could inhibit the proliferationof PC3 cells and growth of its xenografted tumors. Curcumin, the active ingredient of turmeric, has shownanti-cancer properties via modulation of a number of different molecular regulators. Here we investigatedwhether Id1 might be involved in the anti-cancer effects of curcumin in vivo and in vitro. We firstly confirmedthat curcumin inhibited cell viability in a dose-dependent fashion, and induced apoptosis in PC3 cells, associatedwith significant decrease in the mRNA and protein expression of Id1. Similar effects of curcumin were observedin tumors of the PC3 xenografted mouse model with introperitoneal injection of curcumin once a day for onemonth. Tumor growth in mice was obviously suppressed by curcumin during the period of 24 to 30 days. BothmRNA and protein levels of Id1 were significantly down-regulated in xenografted tumors. Our findings pointto a novel molecular pathway for curcumin anti-cancer effects. Curcumin may be used as an Id1 inhibitor tomodulate Id1 expression.