A method of fabricating a read only memory. After forming bit lines and word lines in a substrate, a coding process is performed. A photoresist layer is formed on the substrate while performing the coding process. The photoresist layer covering a part of a first channel region under the word line is exposed, and then the photoresist layer covering a part of a second channel region under the word lines is exposed. A development step is performed to remove the photoresist layer that has been exposed. Using the remaining photoresist layer as a mask to perform an ion implantation, a coding area is formed in the first channel region and the second channel region. The photoresist layer is removed.