A curved electrode structure includes a piezoelectric material layer, a first conductive layer, a first protection layer and a second conductive layer. The piezoelectric material layer is disposed between the first conductive layer and the second conductive layer. The first conductive layer is disposed on the piezoelectric layer. Each of the first conductive layer and the piezoelectric material layer has a first contact surface. The two first contact surfaces are both circular shaped. The first protection layer is disposed on the conductive layer. Each of the first protection layer and the first conductive layer has a second contact surface. The two second contact surfaces are both circular shaped.