Today, the incredible growth i the use of electronic device that require DC power supplies leads to employing more and more DC-DC converters. The main function of these converters is to transfer the power from the power grid to the electronic equipment. The rectifier of the converter, which is placed at the input of the converter, is consists of a bridge diode and a capacitor. The input capacitor is charged through input bridge diode at the peak of the line voltage, which causes a pulsating input current. This pulsating current causes power grid problem, and pollutes line current with harmonic components. Therefore, in order to limit the injected harmonics and to achieve high power factor (PF), some regulations must be met. As the power usage increases, the standards are more stringent. To meet these standards, electronic equipment designers should use power factor converters (PFC), which are located after the bridge diode in order to shape the input current. PFCs are divided into several categories and Single-Stage PFC is one of these structures. Single-Stage PFCs are best suitable for low power application, due to their reduced number of electrical components. In this thesis, basic concepts and some indicators for measuring the undesirable harmonics are introduced at first. Then a review of various structures is presented in the second chapter. In Chapter III, the first proposed Single-Stage power factor structure is presented, which uses the buck converter as PFC stage. This converter benefits from low voltage stress of the bulk capacitor owing to the use of the Buck converter. Also, despite the Buck converter, the proposed structure doesn’t require a floating-source pulse to trigger the switch. Another advantage of the proposed converter is that the issue of distortion of zero crossing for the input current of the Buck converter can be almost completely eliminated by using a compensator circuit with the minimum number of elements. In section IV, another Single-Stage PFC structure is proposed, in which the soft switching conditions is provided in addition to having the advantages of the previous converter. Thus, the efficiency of the converter is greatly increased. In other words, both proposed converters achieve to overcome the disadvantages of the buck converter, while still maintaining the benefits of the Buck converter, making them an ideal option for power factor correction. In section V, the two proposed structures are compared with the converters discussed in the second section, and their advantages and disadvantages are compared from different aspects. Key Words : Power factor correction, single stage, soft switching