Abstract: A classical Hill cipher breaks the plaintext into blocks and multiplies each block to a key matrix to obtain the cipher text. But it inclines to known plaintext. To strength the Hill cipher, a novel modification is performed in this study. A double protection has been given for the Hill cipher with a single private key matrix. First protection is given by modified key matrix Km, obtained from the private key matrix with some arithmetic operation and the second protection is given before transmitting the cipher text with respect to the ‘t’ matrix. Without the private key matrix and the modified key matrix, cipher text cannot be decrypted. The proposed Double Guard Hill cipher is suitable for Wireless Sensor Networks as it is capable of encrypting 128 ASCII values.