Chemical investigation on the ethanol extract of stem of Spiraea formosana has resulted in the isolation of a new diterpenoid alkaloid, spiraeaine A (1), together with seven known compounds, bakuchiol (2), p-hydroxybenzaldehyde (3), nonadecyl ferulate (4), glutinol (5), beta-sitosteryl glucoside (6), mixture of beta-sitosterol (7) and stigmasterol (8). Their structures were determined by the spectral analyses and by comparison with the literature values.