Abstract: The fragmentation pathways of 28-homobrassinolide(28-h-BL) and 28-epihomobrassinolide(28-eh-BL) were investigated by tandem mass spectrometry(MSⁿ, n＝1, 2, 3, 4). The proposed fragmentation mechanism could be potentially used to identify and distinguish brassinosteroids and their isomers.