In this study, a tracer gas technique was used to measure ventilation effectiveness in a thermal environmental chamber simulating an under-floor air conditioning system. A tracer gas of $SF_6$ was injected in a supply duct using step-up and step-down methods. Local mean ages and room mean ages were calculated from the measured concentrations under isothermal and cooling conditions with and without diffusers. Ventilation effectiveness is found to be higher in cooling ventilation operations than in isothermal operations. Results also show that ventilation effectiveness is not significantly affected by a diffuser.