Abstract: OBJECTIVE To establish a method based on mass spectrometry structural assignments for identification of sildenafil analog added illegally in herbal product. METHODS A high-performance liquid chromatography-diode array detection (HPLC-DAD) method and a liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) method were applied. RESULTS Identification of hydroxyhomosildenafil, an analog of sildenafil was achieved by comparing UV spectrum and mass fragmentation results of the samples with those of sildenafil. Three batches of Suoyang Gujing Wan, a dietary supplement, were found containing hydroxyhomosildenafil. CONCLUSION Based on structural assignments of mass fragments, analog of sildenafil could be rapidly and accurately determined in herbal product.