Abstract: OBJECTIVE To identify a new tadalafil related compound adulterated in a dietary supplement. METHODS A new tadalafil related compound was detected in a routine screening for compounds adulterated in a dietary supplement by HPLC-DAD. The target compound was isolated by preparative high performance liquid chromotography. Its molecular weight and structural fragments were obtained by HPLC-MS/MS. ¹H-NMR spectrum and ¹³C-NMR spectrum were obtained by means of nuclear magnetic resonance. Its structure was identified by analysis of all these data and related textual comparison. RESULTS A new tadalafil related compound named diethylaminopretadalafil was detected in a dietary supplement. CONCLUSION The compound is not in the current inspection standard lists, which includes 11 target compounds. It is necessary to study this compound.