An integrated transformer includes a primary inductor and a secondary inductor wherein the primary inductor includes a B turns spiral winding formed by a first metal layer and an A turns winding formed by a second metal layer, wherein the A turns winding formed by the second metal layer and the innermost turns of the B turns spiral winding formed by the first metal layer are substantially overlapped; and the secondary inductor includes a C turns winding at least formed by the second metal layer, wherein the C turns winding formed by the second metal layer of the secondary inductor and a portion of the winding formed by the first metal layer of the primary inductor are substantially overlapped, wherein A is not bigger than B, and A is not bigger than C.