Four new cembrane diterpenes (1-4) and fifteen known cembranoids (5-19) were isolated from an Okinawan softcoral Lobophytum crassum. The structures of these four new cembranoids were determined on the basis of spectroscopic evidence. In particular, the absolute stereochemistry of 1, 2, 5 and 6 were elucidated by the application of the modified Mosher's method and circular dichroism (CD) spectral data