A source driver for driving at least one sub-pixel is disclosed, in which the source driver includes a gamma voltage generator and a digital to analog converter. The gamma voltage generator generates a plurality of gamma voltages, in which the gamma voltage generator includes a first gamma resistor string and an operation circuit. The first gamma resistor string includes a plurality of resistors electrically connected serially for dividing a first gamma reference voltage and a second gamma reference voltage into the gamma voltages. The operation circuit optionally adds increments to the gamma voltages according to a timing control signal, wherein the increments are the same when the gamma voltages are added. The digital to analog converter selecting one of the gamma voltages generated by the operation circuit as a driving voltage based on received digital pixel data.