Abstract: In this paper the concept of microfunction is first defined and then a fault model for the instruction sequencing function of microprocessors is introduced. According to the features of ALU, a corresponding fault model is given. In our testing algorithm for microprocessors, a modified finite state machine defined in this paper is first checked, and an initialization sequence for microprocessors is obtained. Then, the testing procedures of the instruction sequencing function and the data transfer function are derived. It is proved that this two testing procedures must be the testing procedures of microprocessors, thus simplifying the test generation of microprocessors. Finally, the test procedures of the instruction sequencing function presented in this paper are compared with the method in 2.