In the newly developed laser-illuminated integrating sphere source, speckle in the image from the source is effectively removed by rotating diffusers in the sphere to improve the uniformity and stability of the radiation field. The specific relationship between the speckle removal effect and the rotation frequency of the diffusers is evaluated. Speckle contrast of the exit port image is calculated under different rotating speed and different CCD integrating time. We have checked that the speckle noise is reduced to a negligible level by rotating the diffuser at very low frequencies. The speckle contrast is approximately 30% when the diffusers are motionless, and it is reduced to 5% when the speckle is removed.