In this paper, we analized a monoalphabetic substitution ciphers. First of all, we gave defnitions and results from algebra and number theory, usefull fot successfully reading the content of the paper. We defined the monoalphabetic substitution ciphers. Through the different types of examples we illustrated encryption and decryption of plain text. Moreover, we mentioned some versions of the monoalfabetic codes such as Hebrew code, Caesar cipher, Afine cipher, Pigpen code and Polybius square. Each of them is confirmed by various type of examples. Besides that, we explained the homophonic substitution cipher and some of its variants. At the end, we mentioned the code used by the serial killer Zodiac, whose ciphertext has not been decrypted for decades.