The qualification of probe heads before the measurements performed with their usage is now a standard procedure. It is used for correction of probe head's and measuring tip systematic errors. The calibration sphere which is used for this process should have adequately small form errors. However, it is not always possible to use a highly accurate spheres and a common practice is to use for a long time, a sphere that was provided by the manufacturer of the CMM. This is why, the authors have tried to answer the question about the impact of the calibration sphere inaccuracies on the results of probe head qualification process.