Abstract The object of this study was to evaluate the energy performance of an indirect and direct evaporative cooler assisted 100% outdoor air system (IDECOAS), of which a pilot unit was installed in a campus building, during its actual operation in cooling and intermediate seasons. A conventional variable air volume (VAV) system was also installed for comparison with the proposed system. The IDECOAS operated in two operation modes (i.e., single-stage and two-stage operation) depending on the seasonal conditions (i.e., intermediate season and cooling season). In addition, the effectiveness of the indirect evaporative cooler (IEC) and the direct evaporative cooler (DEC) were monitored by measuring the variation of air temperature and relative humidity at each measurement location. The fan and cooling coil energy consumption were also measured by installing watt meters on each device. It was concluded that the IDECOAS operating in the two-stage mode in the intermediate season shows a 51% energy saving over the conventional VAV system. However, the proposed system may consume 36% more operating energy than the conventional VAV system during the cooling season. This may be caused by limited cooling performance of the IEC in hot and humid climate.