Based on the warm tension test and metallurgical microstructure observations, the change of the microstructure of B340/590DP dual-phase steel, especially the change of grain size in different strain rates and temperatures, was studied in this paper. The results of the experiment show that: in the warm tension tests of the dual-phase steel, both the dynamic recovery and dynamic recrystallization affect the grain size, and the influence of dynamic recrystallization is stronger. In different rates of extension, the microstructure all has the minimum grain sizes in the medium term of the dynamic recrystallization. In the medium term of the dynamic recrystallization under different rates of extension, the minimum grain sizes increase as strain rate drops. The temperature in which the minimum grain size forms decreases as strain rates drops.