In order to study roundness error in centreless grinding, key factors affecting grinding process were discussed, including stability, grinding parameters, grinding wheel and control wheel. The measures to improve roundness error were provided. An analytical investigation on rounding mechanism in time domain was carried out. In the investigation, nonlinear aspects were taken into account, and the rounding process was studied by combining workpiece profile and interaction from grinding wheel, workrest and control wheel to workpiece. The comparison between experimental results and theoretical results demonstrates that the model can be used to predict rounding process accurately.