A recessed channel array transistor may include a substrate, a gate oxide layer, a gate electrode and source/drain regions. The substrate may have an active region and an isolation region. A recess may be formed in the active region. The gate oxide layer may be formed on the recess and the substrate. The gate oxide layer may include a first portion on an intersection between a side end of the recess and a sidewall of the active region and a second portion on a side surface of the recess. The first portion may include a thickness greater than about 70% of a thickness of the second portion. The gate electrode may be formed on the gate oxide layer. The source/drain regions may be formed in the substrate. Thus, the recessed channel array transistor may have a decreased leakage current and an increased on-current.