Heat pipes (HPs) are being widely applied in the cooling systems of electrical machines with remarkable cooling effectiveness reported in the existing literature. However, attention is rarely paid to the effect of HP inclination angle on its thermal performance and resulted non-uniform temperature distribution issues in electrical machines. In this article, a dedicated experimental platform is established and the equivalent thermal conductivities of HPs at different inclination angles are accurately measured by experimental tests. On top of that, a 3-D thermal model based on a full-size stator-winding assembly where HPs are evenly inserted into the winding is built and the temperature distribution across the whole winding is comparatively studied under different cooling methods and thermal loads. The results show that the equivalent thermal conductivities of the HPs at different inclination angles vary more than five times, which leads to a maximum temperature difference of up to 4.317°C under the liquid cooling method.