A thin film transistor includes a first blocking layer disposed on a substrate, and an active pattern disposed on the first blocking layer. The active pattern includes a source region, a drain region, and a channel region disposed between the source region and the drain region. The thin film transistor further includes a gate electrode disposed on the active pattern. The channel region corresponds to a portion of the active pattern overlapped by the gate electrode. The thin film transistor additionally includes a source electrode connected to the source region, and a drain electrode connected to the drain region. The active pattern includes a first part and a second part. The first part partially overlaps with the first blocking layer, and the first part and the second part have different thicknesses from each other.