Abstract: An optical waveguide circuit substrate includes a circuit board and an optical waveguide structure disposed on a lower surface of the circuit board. A plurality of pads are disposed on an upper surface of the circuit board. The optical waveguide structure includes a first cladding layer, a second cladding layer, a core layer and a reflective layer. The core layer has an imprinted opening of which an aperture gradually increases from the first cladding layer to the second cladding layer. A first portion of the second cladding layer fills the imprinted opening and has a connection surface. The reflective layer is located between the core layer and the first portion, and an angle between the reflective layer and the connection surface is in a range from 44 degrees to 46 degrees. An orthographic projection of the reflective layer on the upper surface is located between the pads.