In cryptography, a substitution block cipher is a method of encryption by which units of plain text are replaced with cipher text according to a regular system. The receiver deciphers the text by performing an inverse substitution. If the cipher operates on single blocks, it is termed as simple substitution block cipher. The proposed method that considers a random matrix which is chosen from the length of the given input text. Here the input text is stored in the matrices which are selected with minimum cost. There on the series of operations are performed and cipher text is produced, in decryption inverse matrix is calculated and the operations are performed in order to generate the plain text. As we are using random matrix and then the inverse matrix calculation is performed which adds more security for cracking the key. Thus, the cipher text obtained is very difficult to be broken without knowing the key, which provides high security.