In the present study rotating vaneless diffusers are considered to minimize energy losses associated with diffusion and to improve the operating range of the compressor. Depending on the speed of rotation the diffusers are classified into two types forced and free. Simulations using ANSYS CFX 17.2 are performed with free rotating diffusers, in which diffusers are rotated at fraction of impeller speed. The performance of rotating diffuser with the stationary diffuser is compared using several performance characteristics such as total pressure ratio, static pressure recovery coefficient, coefficient of torque and stagnation pressure loss coefficient are plotted in each case. Simulations are performed at different speed ratio over a range of 0.25-0.95. Among all the cases vaneless diffuser with speed ratio 0.75 has optimum performance in terms of performance characteristics and velocity streamlines are uniform with lower Mach number compared to the stationary vaneless diffuser. © 2021 Author(s).