Nowadays the ac/dc converters are widely used in many off-line power supplies. The increasing amount urges researchers to develop more efficient, smaller size, and low cost ac/dc converters. Traditionally, an ac/dc converter consists of a diode-bridge rectifier followed by a bulky capacitor and a high-frequency dc/dc converter. This kind of converter inevitably introduces highly distorted input current, resulting in a large amount of harmonics and a low power factor. paper proposes a single-stage high-power-factor ac/dc converter with symmetrical topology. The circuit topology is derived from the integration of two buck-boost power-factor-correction (PFC) converters and a full-bridge series-resonant dc/dc converter. Switch-utilization factor is improved by using two active switches to serve in the PFC circuits. A high power factor at the input line is assured by operating the buck-boost converters at discontinuous conduction mode. With symmetrical operation and elaborately designed circuit parameters, zero-voltage-switching on all the active power switches of the converter can be retained to achieve high circuit efficiency. The operation modes and design equations for the circuit parameters are proposed.