Bioassay-guided fractionation of a $CH_2Cl_2$-soluble fraction of the stems of Catalpa ovata led to isolation of a new naphthoquinone, 4-hydroxy-2-(2-methoxy-3-hydroxy-3-methyl-but-1-enyl)-4-hydro-1H-naphthalen-1-one (10), together with nine known compounds, catalponol (1), catalponone (2), catalpalactone (3), ${alpha}$-lapachone (4), 9-hydroxy-${alpha}$-lapachone (5), 4,9-dihydroxy-${alpha}$-lapachone (6), 9-methoxy-${alpha}$-lapachone (7), 4-oxo-${alpha}$-lapachone (8), and 9-methoxy-4-oxo-${alpha}$-lapachone(9). The structures were elucidated on the basis of spectroscopic analyses. The inhibitory effects of these isolates on lipopolysaccharide-induced NO synthesis in RAW 264.7 cells were evaluated. Among them, catapalactone (3), 9-hydroxy-${alpha}$-lapachone (5) and 4,9-dihydroxy-${alpha}$-lapachone (6) exhibited potent inhibitory effects, with $IC_{50}$ values of 9.80, 4.64 and $2.73;{mu}M$, respectively.