The notable features for inactivation of Ca_v3 channels are fast inactivating rate and strong voltage-dependence. We have investigated the molecular basis for determining the voltage-dependence of inactivation for Ca_v3.1, focusing on domain Ⅰ and Ⅱ. We made chimeras between Ca_v3.1 and Ca_v1.2. Chimeras were expressed in oocytes and currents were recorded by voltage clamp. For domain Ⅰ, replacement of S1～S4 or S5～S6 shifted the steady state inactivation curve significantly. These changes were mainly or partially caused by activation-inactivation coupling, rather than molecular modification. Replacement of domain Ⅱ shifted the inactivation curve significantly and these changes refer to molecular modification, indicating that domain Ⅱ contributed to the voltage-dependence of inactivation for Ca_v3.1. Furthermore, both voltage sensor region S1～S4 and pore region S5～S6 in domain Ⅱ were also involved, but Ⅰ-Ⅱ linker has no contribution. In addition, we found that the Ⅰ-Ⅱ linker and S5～S6 in domain Ⅰ contributed strongly to inactivation rate for Ca_v3.1, while S1～S4 in domain Ⅰ and Ⅱ was not involved. Taken collectively, our results suggest that domain Ⅱ plays a key role in determining the voltage-dependence of inactivation for Ca_v3.1, which was different from the molecular determinants for inactivating rate and for voltage-dependence of activation.