This thesis presents a LLC resonant adapter consisting of a front-stage power factor correction circuit and a second-stage half bridge DC-DC converter. The PFC circuit works in discontinuous conduction mode to correct power factor. The second-stage LLC resonant circuit has high conversion efficient characters of zero voltage switching on primary power switches and zero current switching on secondary rectifiers. The experiment result shows that the adapter has the highest efficiency at 93.2% and PF is above 0.96 as full load.