The sinking of the Cheonan is important to give South Korea the opportunity to assess security environment and defense posture in the Korean peninsula. South Korea underestimated operational abilities of North Korea's submarine and the attack possibilities. Cheonan incident have caused South Korea to reassess North Korea's asymmetric threats and to thoroughly assess our security posture. Political aspects is that Cheonan Incident has caused post-Cold War in Northeast Asia and made the relation between both Korea worsened. However, our military should reflect the lessons learned from the Cheonan on defense reform. South Korea must arrange defense system and defense policies after North Korea's Cheonan attack. This paper distinguishes 10 kinds of North Korea's asymmetric power and suggests methods to cope with the North's attack. There is no military advantage to divide North Korea's threat into asymmetric threat or symmetric threat. Biased perspectives on security that symmetric threat is pair and asymmetric threat unpair bring deadlocks in international community to end. Under the circumstance North Korea has threaten continuously, the effective policies to handle threats are needed. The efforts to reach National consensus that North Korea is afraid of the South Korea's retaliation could stop other threats from the North such as Cheonan incident rather than that South Korea endure military revenge against the North.