Accurate air-fuel ratio control is required for good engine performance and low emission in diesel dual fuel engine. Two actuators directly affect the ratio are the air throttle and the EGR valve. Maximum air throttle opening is favorable to minimize pumping loss, and the EGR valve opening should follow closely the values in a well-tuned map to minimize emission. In the past, the two actuators were either controlled separately or simultaneously to achieve the air-fuel ratio set point without much consideration on the actuators' opening positions. We proposed a logic that alternated between actuating the air throttle and the EGR valve to maintain optimum air throttle opening. Since each actuator was controlled one at a time, the overall control system was simplified, yet any advanced controller could be applied to increase the accuracy of each actuator. Experiments on four-cylindrical diesel-dual-fuel engines showed that the air throttle opening was optimized at all time, whereas the EGR valve opening followed closely the values in a well-tuned map. The air-fuel ratio was also accurately regulated with widest range possible. Both new-European-driving-cycle and set point changes tests were performed on engine and chassis dynamometers, which demonstrated the effectiveness of the proposed method.