A testkey structure including the following components is provided. A fin structure is disposed on a substrate and stretches along a first direction. A first gate structure and a second gate structure are disposed on the fin structure and stretch along a second direction. A first common source region is disposed in the fin structure between the first gate structure and the second gate structure. A first drain region is disposed in the fin structure at a side of the first gate structure opposite to the first common source region. A second drain region disposed in the fin structure at a side of the second gate structure opposite to the first common source region. A testkey structure is symmetrical along a horizontal line crossing the first common source region. The present invention further provides a method of measuring device defect or connection defect by using the same.