In a thin film transistor substrate, an active pattern of a thin film transistor includes a lower semiconductor pattern and an upper semiconductor pattern that are patterned through different process steps. The lower semiconductor pattern defines a channel area of the thin film transistor, and the upper semiconductor pattern is connected to a side portion of the lower semiconductor pattern and makes contact with the source electrode and the drain electrode. An etch stop layer is formed on the lower semiconductor pattern corresponding to the channel area, and the etch stop layer is formed through the same patterning process as the lower semiconductor pattern. Also, an ohmic contact pattern is formed on the upper semiconductor pattern, and the ohmic contact pattern is formed by the same patterning process as the upper semiconductor pattern.