The U.S. has been building the security umbrella in Asia-Pacific after the World War II, from Japan and South Korea in the north, to Philippine, Thailand, Australia and New Zealand in the south, to protect its homeland security and interests in Asia-Pacific from communist threat like Communist China. When the Cold War ended which saw the decline of USSR as a world power, the U.S. security umbrella continues but it now faces a new challenge posed by the rise of China. While Australia is part of the security umbrella of U.S., it is quite different from Japan, South Korea, Philippine and Thailand. Australia is a state with cultures and values similar to the West but is geographically located in Asia-Pacific. It has huge territory that ranks top 6 in the world but sparsely populated with only around twenty one million people. Trade contributes significantly to its economic development due to its relatively small population and abundant resources. China's influence is rising after its economic reform. China has been the most important trade partner of Australia since 2007. However, when Australia seeks a wider bilateral interaction with China, it is expected that this will have an impact on the Australia-United States bilateral relations. The concept of a ¡§Strategic Triangle¡¨ has been used in this dissertation to observe the changes of the Australia-United States-China relationship in different stages. The national interests approach is applied to interpret the importance of relationships of Australia-United States and Australia-China to Australia. Therefore, the contents of Australia¡¦s national interests and Australia's foreign policies are explored first. Then, the bilateral relationship between U.S.-China is examined for its impacts on Australia¡¦s choices between U.S. and China. The Australia-U.S. relationship and Australia-China relationship are also examined respectively to understand the rationale behind Australia's approach toward U.S. and China. Finally, Australia's strategies are reviewed and the possible strategic choices for Australia are discussed. The strategic choices made by Australia could be interpreted from the Australia-U.S.-China triangular relationship.