In this era of information technology and ecommerce security is the key aspect while transmitting confidential information over insecure network like internet. In order to protect the information, various security mechanisms are used. Cryptography is a technique of protecting secure information from unwanted individuals by converting it into unintelligible form. It is an emerging technology in the area of network security. In this paper, critical analysis of some of the symmetric key block cipher algorithms DES, 3DES, AES, IDEA, Blowfish and RC2 is presented.