If the door opening problem occurs in side crash, it results in raising the possibility of passenger ejection and serious injury. Therefore, for the sake of passenger’s safety, various research on preventing door opening during side impact test is conducted. Even though there are many causes for door opening, this study dealing with inertia effect caused by impact energy. Until now, there have been two classical methods for preventing the door opening. One is increasing the balance weight and spring force. That counterbalances the outside handle’s opening direction moment. The other is the application of the blocking lever. That prohibits the outside handle’s movement mechanically in the event of side crash. Recently, it is shown that the trend of crash test enhances the safety of the passengers by increasing the speed and weight of MDB. As a result, the impact energy transmitted to the vehicle increases. However, there are limitations in increasing the balance weight and the spring force because it brings side effects such as hard door operational feeling. The purpose of this study is to develop the new mechanism that prevents door opening by strengthening the robustness of the operation of the blocking lever. The concept of the new mechanism is expressed as follows. 1) If the inertia effect is applied, the blocking lever should blocks outside handle’s movement. 2) After the inertia disappears, the blocking lever should be locked to block position. 3) After the handle operated, the blocking lever goes back to its initial position. The effectiveness of the new mechanism is verified by mock-up sample and the test rig. Also, not only the advantages such as weight reduction of the vehicle by removing the balance weight but also easy control of the door operational feeling by decreasing the spring force could be obtained.