Eighteen previously undescribed lanostane triterpenes and thirty known analogues were obtained from the fruiting bodies of Ganoderma resinaceum. Resinacein C was isolated from a natural source for the first time. The structures of all the above compounds were elucidated by extensive spectroscopic analysis and comparisons of their spectroscopic data with those reported in the literature. Furthermore, in an in vitro assay, Resinacein C, ganoderic acid Y, lucialdehyde C, 7-oxo-ganoderic acid Z(3), 7-oxo-ganoderic acid Z, and lucidadiol showed strong inhibitory effects against alpha-glucosidase compared with the positive control drug acarbose. The structure-activity relationships of ganoderma triterpenes on alpha-glucosidase inhibition showed that the C-24/C-25 double bond is necessary for alpha-glucosidase inhibitory activity. Moreover, the carboxylic acid group at C-26 and the hydroxy group at C-15 play important roles in enhancing inhibitory effects of these triterpenes.