A novel HVAC system combining natural ventilation with diffuse ceiling inlet and thermally activated building systems (TABS) has the ability to fulfill the requirements of cooling and ventilation in future Danish office buildings. In order to study the cooling performance of this system, a test chamber is constructed in a way to represent the characteristics of an office room. Twenty cases are tested under steady-state conditions, including ten cases without ceiling panel and ten cases with ceiling panel. An energy balance analysis shows that the tests have quite good accuracy, with an error of less than 10%. Both the cooling capacity of TABS and the influence of ceiling panel are investigated. The U-value of TABS water side to the room side is almost constant, but the effectiveness of TABS decreases with log mean temperature difference (LMTD) for both cases with and without ceiling panel. The radiant heat transfer coefficient of TABS is reduced by the ceiling panel whereas the convective heat transfer coefficient increases with the ventilation rate and the inlet air temperature. Experimental data is used to evaluate the thermal performance of this system, and it is also beneficial to the design of this system.