A thin film transistor array panel includes a substrate, a light blocking film disposed on the substrate, a buffer layer covering the light blocking film, and a channel region disposed on the buffer layer. A source region and a drain region are disposed in the same layer as the channel region. A gate insulating layer is disposed on the channel region, and a gate electrode overlaps the channel region, with the gate insulating layer interposed between the gate electrode and the channel region. A passivation layer is disposed on the gate electrode, the source region, the drain region, and the buffer layer. A source electrode and a drain electrode are disposed on the passivation layer, wherein the channel region, the source region, and the drain region comprise an oxide semiconductor, and wherein a carrier concentration of the source region and the drain region is larger than in the channel region.