Error separation technique is the one of the best ways for accurate and precise determination of roundness form. The multi-step technique is an important method of roundness error separation. Many national metrology institutes (NMIs), industrial calibration laboratories and research institutes have been using the multi-step method for high precision and high accuracy in roundness measurement. Number of step used by various institutes is different. In this paper, effect of number of step employed for the error separation was investigated. The results of these experiments successfully indicate that number of step used in multi-step method plays an important role to the accuracy level of the roundness measurement result. In order to achieve accuracy at the ultra-high level, at least 10 steps should be employed using multi-step method to satisfy the geometrical form of roundness measurement.