The performance of vapor chambers, also known as flat heat pipes, was studied experimentally. Five vapor chambers with different vapor space heights were intentionally fabricated and tested. The hydraulic diameter ratio was created to be the dimensionless parameter to represent the variation of the vapor space height. The parameter was further used to derive the optimum vapor chamber height. It was found that the vapor chamber achieved the optimum design when the hydraulic diameter ratio of non wick vapor space to chamber interior was greater than 0.6. It is believed that the results are valuable for future vapor chamber design.