A metal mask and an inspecting method thereof are provided for improving quality standard detection. The metal mask has a first and a second long side and plural pattern regions. The method includes the followings steps Based on the pattern regions adjacent to the first and second long sides, a first and a second reference straight line adjacent to the first and second long sides respectively are defined. Then, a first maximum offset length between the pattern regions and the first reference straight line is measured. A second maximum offset length between the pattern regions and the second reference straight line is measured. When a difference between the first and second maximum offset lengths is less than or equal to 20 μm, the metal mask is determined to meet an inspecting standard.