Micro damage on metal surface will cause the change of conductivity. By accurately measuring conductivity, quantitative assessment of micro damage can be achieved accurately. Planar electromagnetic sensor could be used to obtain conductivity through measurement database which relates sensor response to conductivity, the property of material under test. Measurement grid is graphical database, and its features have significant influences on measuring precision and speed. However, there are few studies on grid features of planar electromagnetic sensor. This paper analyses how the conductivity error is influenced by three features of grid unit, and presents the relationship between them, which could be used to optimize the database.