In today’s Intet computing world, illegal activities by crackers pose a serious threat to computer security. It is well known that computer viruses, Trojan horses and other intrusive programs may cause severe and often catastrophic consequences. This paper proposes a novel secure computer architecture based on security-code. Every instruction/data word is added with a security-code denoting its security level. Extal programs and data are automatically added with security-code by hardware when entering a computer system. Instruction with lower security-code cannot run or process instruction/data with higher security level. Security-code cannot be modified by normal instruction. With minor hardware overhead, the new architecture can effectively protect the main computer system from destruction or theft by intrusive programs such as computer viruses. For most PC systems, it includes an increase of word-length by 1 bit on registers, the memory and the hard disk.