Development of electronic equipment and switching power supplies has vastly increased the usage of diode rectifiers with capacitive load in order to provide the required input power. Since diode rectifiers are nonlinear loads, the input current contains harmful harmonics which results in severe problems, such as additional conduction losses of transformers, distortion of line voltage, high total harmonic distortion (THD) and non- sinusoidal line current. To limit the current harmonics produced by these nonlinear devices, standards like International Electro technical Commission (IEC) 1000- 3- 2 are defined. Thus, adding power factor correction (PFC) cell at the input of switching power supplies is necessary. Passive power factor correctors are normally used in high power line applications. But they are heavy and bulky, and the output voltage is unregulated. So active power factor correction converters are proposed. Two- stage PFC uses a PFC converter at first stage after the diode rectifier to shape the input current, and at the second stage a DC- DC converter is used to regulate the output voltage. But they have low efficiency because of twice power processing. To reduce cost and complexity of two stage PFC converter, single stage and parallel PFC converters are introduced. In parallel structure, power flows through two paths. The main part of power is transferred directly to load for PFC purpose, and the other part is processed twice in order to regulate the output voltage. In this thesis, various kinds of parallel PFC converters are investigated. Then, a parallel soft- switching power factor correction circuit is proposed. This converter is based on an active clamp flyback circuit. Power factor correction is achieved with minimum double process of power, while providing ZVS condition for the main switch and the auxiliary switch. The proposed converter processes 68% of the input once and the 32% of power is processed twice, so the efficiency is improved in comparison with other PFC converters. The proposed converter operates in DCM condition and it has intrinsic power factor correction characteristic. Thus, there is no need for extra control loop to shape the input current. The active clamp circuit limits the voltage spike of the main switch due to the transformer leakage inductance. The proposed new soft switching parallel PFC converter is analyzed and the simulation and experimental results are presented to verify the theoretical analysis. Keywords: Power Factor Correction (PFC), Parallel Power Factor Converter, Soft Switching, Flyback Converter, Active Clamp Circuit