A new ent-abietane diterpenoid, $3{alpha}$, $6{eta}$-dihydroxy-7,17-dioxo-ent-abieta-15(16)-ene (1), and three known ent-kaurane diterpenids, kamebacetal A (2), kamebakaurin (3), and excisanin A (4), and a known triter-penoid, ursolic acid (5), were isolated from the aerial parts of Isodon inflexus. Their chemical structures were determined by extensive analysis of spectroscopic data including 1D- and 2D-NMR experiments. All isolates (1-5) were evaluated for their potential to inhibit LPS-induced nitric oxide production in RAW264.7 cells. Of these, compounds 1-4 inhibited the production of NO with $IC_{50}$ values ranging from 1.0 to $26.5{mu}M$.