A thin film transistor panel includes a substrate, a light blocking layer on the substrate, a first protective film on the light blocking layer, a first electrode and a second electrode on the first protective film, an oxide semiconductor layer on a portion of the first protective film exposed between the first electrode and the second electrode, an insulating layer, a third electrode overlapping with the oxide semiconductor layer and on the insulating layer, and a fourth electrode on the insulating layer. The light blocking layer includes first sidewalls, and the first protective film includes second sidewalls. The first and the second sidewalls are disposed along substantially the same line.