Throughout the years, the humidity effect on Partial Discharge Inception Voltage (PDIV) has been investigated revealing the complexity of the interaction between moisture and insulation material. Although the importance of these studies, the Electrical Machine (EM) insulation system experiences combined stresses when in service. Thus, attention is paid to the impact of humidity on PDIV under different thermal aging levels, since the hygroscopicity of insulation material might change during its lifetime. For the experimental investigation, three groups of twisted pairs are prepared and one is unaged while the remaining two are thermally aged at 250°C for 48 and 96 hours respectively. The PDIV and insulation capacitance under sinusoidal excitation is then measured at a fixed 30 °C ambient temperature with three different relative humidity conditions: 30%, 45%, and 60%. The obtained results are discussed and an explanation of the combined effect of humidity and thermal aging is given.