A manufacturing method of a semiconductor package includes the following steps. A supporting layer is formed over a redistribution structure. A first planarization process is performed over the supporting layer. A lower dielectric layer is formed over the supporting layer, wherein the lower dielectric layer includes a concave exposing a device mounting region of the supporting layer. A first sacrificial layer is formed over the supporting layer, wherein the sacrificial layer filling the concave. A second planarization process is performed over the lower dielectric layer and the first sacrificial layer. A transition waveguide provided over the lower dielectric layer. The first sacrificial layer is removed. A semiconductor device is mounted over the device mounting region, wherein the semiconductor device includes a device waveguide is optically coupled to the transition waveguide.