The objective of the present paper is to conduct an experimental work to measure transient thermal/electrical efficiencies in a combined heat and power (CHP) fuel cell generator. An original-designed fuel cell CHP generator comprising a proton exchange membrane (PEM) fuel cell system, a power conditioning system, and a thermal recovery system is constructed. The thermal recovery system recaptures the thermal energy dissipated by the fuel cell stack, while the power conditioning system distributes the electrical power of the fuel cell system. Performance matrices including stack electrical efficiency, system electrical efficiency, thermal efficiency, and CHP efficiency are presented. Particular attention is placed on the variation of transient system efficiencies as the load is changed drastically. The time-averaged system efficiency is then concluded from the statistics of the transient system efficiency. Results show that the time-averaged efficiencies in electrical, thermal and CHP are about 37%, 24%, and 61%, respectively, under 55% electrical power capacity of the generator. The instantaneous CHP efficiency could reach as high as 85% in the loading period.