Abstract This paper considers how the staging of condenser fans can be optimized to improve the efficiency of air-cooled chillers at part load. An experiment on an air-cooled reciprocating chiller confirmed that when all condenser fans are staged to enhance heat rejection airflow, the heat transfer coefficient of the condenser can be optimized, enabling the condensing temperature and chiller power to decrease considerably with less fluctuation. Chiller efficiency, in turn, can be improved by 4.4–40.1%, depending on the load conditions and outdoor temperatures. An algorithm is introduced to compute the number of staged condenser fans based on a set point of condensing temperature for any given head rejection. With this algorithm, the strategy for optimizing the staging of condenser fans is discussed.