XP (eXtreme Programming) has been used for reducing defects and dealing with changing customer requirements effectively. It has been well known for its defect reduction capability. But, software can be made more reliable by means of process improvement. In general, models like CMM/CMMI have been used for software development process improvement. However, these models have some problems to be used with XP because of less number of process artifacts produced by agile processes. Six Sigma on the other hand, provides the quantitative analysis capabilities required to measure and control process performance. Using XP with Six Sigma can provide means of analyzing XP data and systematically improve process performance. In order to prove our point, we map Six Sigma tools to activities involved at each phase of XP and show that Six Sigma technique can be used with XP to measure and improve the performance of XP process. We also suggest a Six Sigma process improvement guideline with examples.