The inhibitory effects of Curcuma longa rhizome-derived materials against nitric oxide (NO) production were assessed. The inhibitory effect (57%) on NO production was evidenced by the methanol extract of C. longa at $1;{mu}g/mL$. In the fractionation of the methanol extract, the ethyl acetate fraction evidenced an inhibitory effect greater than 62.1% at $1;{mu}g/mL$. The active constituent was identified as curcumin. Curcumin exerted potent inhibitory effects of 78.7 and 65.7% at concentrations of 1 and $0.5;{mu}g/mL$, respectively. Furthermore, the inhibitory effect of ar-turmerone was measured as 31.3 and 15.8% at 1 and $0.5;{mu}g/mL$, respectively. The iNOS expression-suppressive effects of curcumin were assessed via western blot analysis. Our results suggest that curcumin and ar-turmerone may prove useful in the development of new types of NO inhibitors.