In order to elucidate the characteristics cavity air flow, exploratory tests are conducted on an unloaded rolling pneumatic tire. A hot-wire anemometer is used to measure the flow velocity inside the cavity. Experimental apparatus is designed to acquire the velocity signals from rotating tire in real time. Hot- wire probe is inserted through the rim and transports the signals to IFA-300 constant temperature anemometer through slip ring assembly. For the case of an unloaded experiment, it is observed that the magnitude of mean velocity in the cavity air shows increasing natures as the inflation pressures and rolling speeds are increased. Furthermore, the increasing rate of velocity for the change of pressure is increased as the tire rotates faster. Experimental results show that viscous force and inertia force play important roles in constructing the flow structure inside the rolling tire.