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