The present paper describes an experimental work to investigate characteristics and generation mechanism of the transonic tone and this work attempts to correlate the transonic tone to flow oscillation from a shock-containing convergent-divergent nozzle. A wall pressure measurement, acoustic measurement and a high speed video camera for Schlieren visualization to visualize shock oscillations are used to investigate the tone generation mechanism. All measurements are set up to get data at the same time. For the conditions of this study the transonic tone takes place in one or two stages or no transonic tone occurs. And it is found that there is close correlation between transonic tone frequency and wall pressure fluctuation or shock oscillation each other when the transonic tone is diffusing from the nozzle.