A method of manufacturing a liquid crystal display device is provided which includes ashing first and second photoresist patterns, whereby a copper oxide film is formed at portions of a data line and a source-drain pattern exposed between the ashed first and second photoresist patterns and between the ashed first and second portions of the first photoresist pattern; deoxidizing or removing the copper oxide film; performing a plasma treatment to change the exposed portions of the data line and the source-drain pattern into a copper compound; removing the copper compound using a copper compound removing solution to form source and drain electrodes below the ashed first and second portions, respectively, wherein the copper compound removing solution substantially has no reaction with the copper group material; dry-etching a portion of an ohmic contact layer between the source and drain electrodes using the source and drain electrodes as an etching mask, the ohmic contact layer formed by patterning the impurity-doped amorphous silicon layer.