After the end of the Cold War some scholars and other experts referred to the theory of “Defensive Realism” when explaining China’s security policy at that time, an approach this study widely However, things have changed since 911, and after a fundamental restructuring in the global balance of power, using the theory of “Defensive Realism” is now not sufficient when analyzing China’s security strategies. This study claims that after 911 the international balance of power changed, mainly because the USA revised their global strategy, thus forcing China to respond by a wide range of policy adjustments, especially concerning security. Using official policy documents and analyzing China’s actions, it was noted that China gradually changed its rather defensive security policy over the time and became a revisionist state, especially in responding to the United State’s new global strategy. This thesis is successfully applying the theory of “Structural Realism” to effectively explain the changes in China’s security policy since 911 and showing that the China after 911 has become a country seeking to expand its power on the world stage, a revisionist state attempting to change the current situation. This thesis uses three indicators, maintaining the status quo, deterrence strategy, and costly signal, to observe and to study if China has made some adjustment of it’s military strategic placement in South-Eastern coast and Taiwan Strait after 911 Attack (from the date of president election of the Republic of China in 2004 up to March 2007) and to discuss if the adjustment is in agreement with the prediction of structural realism. This passage finds out that due to U.S. has changed its global military strategy placement after 911 Attack, the structure of global relationship has been changed and China has seized opportunity to enhance its military and diplomatic power and to expand its power toward the world. Moreover, the attitude of Chinese government has become more positive to the military strategy placement of South-Eastern coast and Taiwan Trait after 911 Attack. Not only China has tried to change it’s current situation with two controversial territories, the South China Sea and Taiwan, but also China has shifted it’s naval strategy from coast-defense oriented to seaward oriented. A significant increasing budget and expense of Chinese military development is able to prove that China is preparing to expanse its military toward the world. Therefore, the passage demonstrates that China is, indeed, adjusting its national security policy and becoming unsatisfied with current situation and turning into a revisionist state gradually. China is becoming a challenger against United States, played an exclusive role administrating the international society.