A phase change chamber with the condensation area smaller than the boiling area can be used in structure optimization of automobile waste thermoelectric generators. The heat flux through the thermoelectric modules increases when adding the chamber between the waste channel and the hot side of the modules, thus increasing the temperature difference between the hot sides and the cold sides of the modules, achieving an increase in output power. In order to investigate heat transfer characteristics of such kind of phase change chamber, three chambers with condensation to boiling area ratios of 1/5, 2/5 and 3/5 were experimentally studied under the boiling heat flux ranging from 4.50 kW/m² to 22.70 kW/m² and the deionized water filling ratio of 33% in this paper. The results show that the boiling heat transfer coefficient increases with increasing heat flux; while the condensation heat transfer coefficients show different trends with increasing heat flux for various area ratios which depends on the droplet distribution. At the same heat flux, the boiling heat transfer coefficients increase with decreasing area ratio, while the condensation heat transfer coefficients decrease with decreasing area ratio. Furthermore, when the boiling heat flux ranges from 5 kW/m² to10 kW/m², the optimum area ratio is 1/5; when the boiling heat flux ranges from 10 kW/m² to 25 kW/m², the optimum area ratio is 2/5.