Air leakage in a rotary heat recovery device in air handling unit (AHU) was studied by a laboratory experiment. The experiment tested a commercial AHU with rotary heat recovery and equipped with various leakage control techniques, e.g. Automatic Leakage Control (ALCTM), rotor speed control (RSC) and purge sector, etc. In the test, exhaust air transfer ratio (EATR ) of the AHU was measured by tracer gas method at two levels of airflow rates in both constant airflow and constant pressure operation modes of the test AHU. The results showed that EATR of the test AHU could be as high as 16% without proper leakage control. The ALC and RSC effectively prevents pollutant transfer from the rotor during both static and dynamic operation of the AHU. The maximum EATR measured from this test was 0.27% when ALC, RSC and purge sector were applied simultaneously for leakage control.