Low evaporator airflow is one of the common faults for rooftop units. It can be caused by dirty filters, evaporator fouling, or loose belts. Low airflow could result in frozen evaporator coils, reduced cooling capacity, and indoor comfort issues. Accordingly, more fan power is consumed as longer operating time is required. With the widespread use of variable-frequency drives (VFDs) on rooftop units (RTUs), low evaporator airflow can be potentially detected by monitoring the fan-power variation. In this paper the principle of fan-power-based detection is introduced first. Then, the detection algorithm is proposed, including development of the baseline and comparison of operational data with the baseline. Finally, the conducted field test is discussed, verifying the proposed methods. The test results indicate that the fan-power-based method can effectively detect low evaporator airflow for rooftop units.