A read only memory including a ROM cell array, a plurality of word lines and a plurality of bit lines and a word line driver. The ROM cell array has a plurality of ROM cells. Each of the ROM cells coupled to corresponding bit line and corresponding word line. The word line driver is coupled to the word lines, and respectively provides a plurality of word line signals to the word lines. Each of the ROM cells is a first type ROM cell or a second type ROM cell. The first type ROM cell includes a first top metal structure and a first bottom metal structure. The first bottom metal structure is electrically isolated from the first top metal structure. The second type ROM cell includes a second top metal structure, a second bottom metal structure, and a connection structure. The connection structure is electrically connected the second top metal structure and the second bottom metal structure.