The aim of this study is to investigate the transient effects of heater on temperature and humidity in an automobile cabin under transient heating period. Interior volume of an automobile is relatively small to other indoor environments, so that temperature and humidity values can rapidly change under highly transient conditions. Studies that are investigating the effects of heater on automobile cabin environment in real car conditions are restricted. Experiments were carried out in a parked car inside a laboratory at nights to prevent the possible wind and solar irradiation effects. In the experiments only plane vents were operated with three different air velocity levels. Temperature and relative humidity differences between air velocity levels were obtained, relations between time, temperature and relative humidity were calculated. From this study it is understood that temperature and relative humidity values inside a car vary directly from air velocity values. The obtained results can be used to improve other thermal comfort studies inside automobiles and also CFD analyses can be validated under transient conditions.