A process of measuring overlay metrologies of wafers, the wafer having a plurality of patterned layers. The process begins with retrieving historical overlay metrologies from a database, and real overlay metrologies of a first group of the wafers are measured. On the other hand, virtual overlay metrologies of a second group of the wafers are calculated with the retrieved historical overly metrologies. The real overlay metrologies of the first group of the wafers and the virtual overlay metrologies of the second group of the wafers are stored to the database as the historical overlay metrologies.