A new pinusolide derivative, 15-methoxypinusolidic acid (1), and another new isopimarane diterpene, ent-isopimara-15-en-3 alpha,8 alpha-diol (2) with three known diterpenes, lambertianic acid (3), isopimara-8(9),15-dien-18-oic acid (4) and isopimara-7(8),15-dien-3 beta,18-diol (5) were isolated from the 90% MeOH fraction of Biota orientalis (L.) ENDL. (Cupressaceae) leaves. Chemical structures of 1-5 were elucidated by analyses of their spectral data, including the two-dimensional (2D) NMR technique. Compound 1 showed significant protective activity against glutamate-induced neurotoxicity in primary cultures of rat cortical cells.