The previous security system was PCI way which has many difficulties for PC novices to use. Moreover the security programs in use are mostly unverified ones as they are using cracks, and are exposed to attacks such as hackers and viruses. Therefore this thesis describes to design the security system of Twofish cryptographic algorithm using USB, which it can be used in general-purpose computers and users can handle it with ease. Users can easily use the security system by using this USB and it is applicable to various security systems that Twofish cryptographic algorithm used in the security system by having variable key length. Also the efficiency of the system can be enhanced as it can perform both encryption and decryption and it has a benefit of downsizing hardware.