The theoretical study of both the conformational distribution and the acid cyclization of two epimeric 5-oxogermacren-6,12-olides in C4 is described. Our results on the stereochemistry of the cyclization products, 5-hydroxyguaian-6,12-olides, agree with experimental results reported previously. Curtin-Hammet principle and Hammond postulate are used to propose reaction pathways consistent with above results. The theoretical results were obtained by using a MM2 program from HyperChem 4.5