A method of copper hillock detecting includes the following steps. A testkey structure is disposed on a substrate, wherein the testkey structure includes a lower metallization layer, an upper metallization layer, and a dielectric layer between the lower metallization layer and the upper metallization layer. A force voltage difference is applied to the lower metallization layer and the upper metallization layer under a test temperature and stress time. A changed sensing voltage difference to the lower metallization layer and the upper metallization layer is detected for detecting copper hillock.