A read only memory (ROM) and an operating method thereof are provided. The read only memory includes: a control circuit, powered by a first power source for outputting a control signal within a first voltage range; a voltage shifter, for expanding the amplitude of the control signal to a second voltage range; a word line driver, powered by a second power source with a voltage which is higher than that of the first power source, for driving one of a plurality of word lines of a read only memory cell array according to the control signal which is expanded to be within the second voltage range; and an input/output circuit, for connecting the plurality of bit lines to read out messages.