This paper studies a method for automated protocol conformance test sequence generation based on formal methods for SDL (Specification Description Language) specifications. We use the I/OFSM (Input/Output Finite State Machine) generated from SDL specifications and a characterizing sequence, called Unique Input/Output (UIO) sequence, is defined for the I/OFSM. The UIO sequence is combined with the concept of Rural Chinese Postman Tour, without reliable reset capability, to obtain an optimal test sequence. To develop this method, first of all, we establish a framework leading to formal testing methodology which can be used to assess the conformance of an IUT (Implementation Under Test) to the behaviours specified in an I/OFSM which is equivalent to corresponding FDT (Formal Description Technique) specification. By this framework, we present a methodology for automatic test generation from FDT specifications. In order to implement and prove the effectiveness of the proposed method, we specify the Q.2931 of B-ISDN protocol in SDL and generate I/OFSM by using our S/W tool.We generate an optimal test sequence by applying our method to this reference I/OFSM. We experienced that the length of the generated test sequence by our method is more effective and shorter than the one generated by UIO method.