During the screening for inhibitors on nitric oxide (NO) production in LPS-activated murine macrophage, RAW 264.7 cells, two coumarins were isolated from chloroform extract of Ponciri Fructus. They were identified as imperatorin (10), phellopterin (11) on the basis of spectroscopic methods. The $IC_{50}$ values for NO formatiom were about $5.4;{um}M$ and $35.0;{um}M$, respectively, and then eleven coumarins were tested for the inhibitory effects on NO production in activated macrophages. All the test coumarins inhibited NO production in concentration-dependent manner and furanocoumarins (6-11) showed much more potent inhibitory effect than simple coumarins. Among the compounds examined xanthotoxin (8) was the most potent inhibitors of NO production $(IC_{50}=1.4;{mu}M)$. Analysis of the structure-activity relationship among these coumarins led to the conclusion that the substitution of C-5 position in furanocoumarins reduces greatly their inhibitory potency although the substitution of C-8 does not almost affect it.