In this paper, six sigma methods were used in bearing ring grinding process improvement. In the improvement, bearing outer ring roundness error was selected as the assessment index, and exsiting problems in grinding process were defined, roundness error of bearing outer ring was measured.One-way analysis of variance was applied to analyze the test results, it indicates that workpiece center height, guide wheel tilt angle and grinding wheel balance are the three main factors that affecting roundness error.According to this analysis, orthogonal experiment was designed. The results show that grinding wheel balance, guide wheel tilt angle and workpiece center height on the impact of roundness error are in descending order. On this basis, a series of measures were carried out to control these three parameters. Uitimately, the grinding process capability index is increased from 0.88 to 1.43, and the target of improvement are achieved.