This paper presents the simulation and transient analysis of conventional and advanced aircraft electric power systems with harmonics mitigation. Complete modeling of aircraft electric power systems is proposed. The conventional aircraft electrical power system is analyzed considering equivalent passive AC and DC loads under transient and steady-state operating conditions. The electric power source is simulated to ensure constant frequency and voltage which meet the aircraft electrical standards for all loading cases. To mitigate the harmonics generated by the converters, passive input filters are designed to keep THD values within the standard limits. Furthermore, the advanced aircraft electrical system is simulated and analyzed under the same electric power source and loads. The results are compared to those of the conventional aircraft electric power system.