The medicinal mushroom Inonotus obliquus has been a folk remedy for a long time in East-European and Asian countries. It has been ascribed to a number of triterpenoids that show various biological activities. In this study, the response surface methodology was employed to optimise the medium composition for triterpenoids production by I. obliquus in shake flask culture. A fractional factorial design was used to evaluate the effects of different components in the medium. Glucose, peptone, yeast powder, and CaCl2 were important factors significantly affecting I. obliquus triterpenoids production. These selected variables were subsequently optimised using steepest ascent method, a central composite design, and response surface methodology. The optimal medium composition was (% w/v): glucose, 5.92; peptone, 0.23; CaCl2, 0.048; yeast powder, 0.12; KH2PO4, 0.1; MgSO4, 0.02. Under optimal conditions, triterpenoids production by I. obliquus reached 5.51%, representing an increase of 1.4-fold compared with that using the basal medium (3.98%).