An AC adapter mounting structure is constructed to include a housing holding a rack. The housing has two vertical pivot rods and two locating holes symmetrically disposed in top and bottom sides of a side opening thereof, and an AC adapter for positioning in the rack inside the housing. The AC adapter has two pivots respectively pivoted in the pivot holes, two short positioning rods for engaging the locating holes of the housing after the pivots are pivoted in the pivot holes and the AC adapter is turned to the inside of the housing, and two stop blocks, which are forced into engagement with respective retaining holes in the rack when the AC adapter is set into the inside of the housing.