Abstract: To characterize the sphericity and the wall thickness uniformity, thick-walled polystyrene (PS) hollow microspheres with inner diameter of 850 m and wall thickness of 25 m were measured by the VMR microscope system, and the effects of sampling methods on the sphericity and the wall thickness uniformity were investigated. For the characterization of a batch of hollow microspheres, the experimental results showed very slight difference when the sample number was no less than 30. For a single hollow microsphere, the average results became stable when the times of the projection torus were at least 3. Therefore, the sample number should be no less than 30 to characterize a batch of hollow microspheres. Moreover, for hollow microspheres with poor wall thickness uniformity or poor sphericity, taking advantage of more than one projection torus benefits improving the reliability of the results.