A new ganoderic acid (GA), 3α,22β-diacetoxy-7α-hydroxyl- 5α-lanost-8,24E-dien-26-oic acid (1), together with four known compounds GA-Mk (2), -Mc (3), -S (4) and -Mf (5), was isolated and characterized from Ganoderma lucidum mycelia. The structure of compound 1 was elucidated on the basis of interpretation of extensive spectroscopic data (HRMS, IR, UV, 1D and 2D NMR). Due to its apparent degradation during preparation procedures, the stability of compound 1 was assessed in several solvents in a short-term study that demonstrated the optimal stability in aproptic environment. A possible mechanism of acid-catalyzed degradation of compound 1 in methanol was proposed, consisting of a fast protonation, followed by a committed step of hydroxyl group removal. In addition, all isolated compounds were tested in vitro for their cytotoxic activities against 95D and HeLa tumor cell lines, with IC50 values ranging from14.7 to 38.5 μM. The results may improve the understanding of chemical stability of GAs and provide valuable information on their separation, analysis and application. © 2012 Elsevier B.V.