Abstract: This paper deals with experimental investigation of unsteady flow in a radial vaneless diffuser.The circumferentially uniform discharge at the exit of the diffuser changes into a periodic circumferentially fluctuating flow state.The experimental measurements show that the properties of rotating stall in the diffuser are very obvious.The results of the investigation indicate that two type of rotating velocity patterns occur successively as the flow rate decreases.Their rotational speeds are different and the pattern with low rotational speed firstly appears.