Modern aircrafts tend to use more electrical power to replace conventional power forms including pneumatic power, hydraulic power and mechanical power on various systems on board for its high efficiency and high reliability. Such aircraft is referred to as the More Electric Aircraft (MEA). To achieve power density requirement for MEA, higher system voltage is preferred. The aimed supply voltage is ±270V while supply voltage for conventional aircraft is only 115V. The new voltage requirement means there is a potential problem for partial discharge to take place in the electrical insulation system for MEA as this voltage has passed the minimum voltage for air discharge to happen given in Paschen's law. Meanwhile, manufactures tend to eliminate the traditional gearbox between flight engines and generators. This new concept is called the More Electric Engine (MEE). Such arrangement would bring a harsh operating environment for the electrical machine especially from thermal stress point of view as the ambient temperature can be as high as 350℃ for the system.Significant challenges have arisen for the electrical machine insulation system on aircraft due to new requirements from both MEA and MEE especially for machine winding insulation. Pure inorganic system made of ceramic or mica has to be used to replace the conventional polymeric system. In this study, high temperature candidate winding wires were examined in both twisted wire form and coil model form. Their detailed insulation layer structures have been checked. Electrical insulation performances for the candidate systems were compared. The effect of different ageing stresses including thermal stress, thermal cycle stress and electrical stress were evaluated for the new system. Ageing mechanisms for the new system were compared with the conventional polymeric system. Applicability of existing standards for electrical machine insulation tests were discussed based on test results. Thermal related stresses have been calculated using the simulation software COMSOL for system design reference.Diffusion process on nickel plated conductor was also studied as such process can affect system power density and has the potential of damaging the insulation system.