A supercritical CO2 (S-CO2) Brayton cycle is expected to have high compactness, simple configuration, and competitive efficiency. These advantages originated from CO2 nonideal gas properties near the critical point. However, the strong real gas effect makes the design challenging. Among components, turbomachinery is most affected by the real gas effects of S-CO2. In order to obtain accurate design of the turbomachinery, accurate loss models are needed. Windage loss is one of the external losses that determines the motor load and is responsible for heat generation due to friction. The overall turbomachinery efficiency is greatly affected by the windage loss especially in an S-CO2 power cycle due to high fluid density and high rotational speed. Therefore, an accurate windage loss model that can reflect the real gas effect of CO2 near the critical point is essential to obtain accurate design of the turbomachinery as well as the power cycle. In this paper, existing windage loss models are compared under S-CO2 conditions first, and the applicability of each model is evaluated. The comparison is to understand how much turbomachinery performance uncertainty can be expected due to the windage loss models and to suggest the way to improve the models.