A method for measuring a thickness of a dielectric layer in a circuit board is provided. The method for measuring the thickness of the dielectric layer includes the following steps. First, a circuit board including at least one dielectric layer and at least two circuit layers is provided. The dielectric layer is between the circuit layers, and the circuit board further includes a test area including a test pattern and a through hole. The test pattern includes at least two metal layers. Next, a measuring device including a main body, at least one light source and a lens module is provided. When the main body is moved into the through hole, the light source emits light to the dielectric layer and the metal layer, and the lens module shoots the dielectric layer and the metal layer to form a captured image. The thickness of the dielectric layer is obtained via the captured image.