In the course of our phytochemical investigation of the liverworts，many kinds of terpenoids having some biological activities were isolated to clarify their chemical characters. Previously，five anti-fungal active diterpenoids have been isolated from the liverwort Odontoschisma denudatum to determine their structures and absolute configurations as a 11，5-bicyclic dolabellane framework. Conformational analysis of the dolabella-3E，7E-dienoid containing a 1，5-undecadiene system offers a great worth in postulating a stereoselective transannular cyclization to the 6，7，5-tricyclic dolastane and 5，8，5-tricyclic fusicoccane skeletons. The endocyclic trans double bonds in medium ring compounds should be almost perpendicular to the average plane of the rings to minimize the transannular nonbonding interaction. Thus，for the dolabella-3E，7E-dienoid four variations of the strain minimum conformers，a a ，a β，a and ββ，are expected by a combination of orientations of the two double bonds. The conformational behavior has been achieved by means of molecular mechanics calculations (MM2) as well as ^<1>H NMR and CD spectrometry. Possible conformers of the 6β-acetoxydolabella-3E，7E-dienoid [1] were judged to be a a form (91.4%，population) and a βform (8.6%，population). It is generally known that an allylic substituent in medium rings controls the conformation of the double bond (A^<1,3>-strain). The major conformer of 6α-hydroxydolabella-3E，7-diene[8] which was transformed from 6 β-hydroxydolabella-3E，7E-diene [2] was suggested to be a mixture of a β conformer and ββconformer in a relation of 11 to 9.