Trust is rare in international relations. States do not trust each other easily in the anarchic world. However, trust is essential for maintaining cooperation and solving crises. According to the reassurance game designed by Andrew H Kydd, states can build up trust gradually by sending costly signals, as sending such signals to manifest a sincere willing of solving a crisis peacefully can effectively avoid a deteriorating relation. The trust building between the United States and North Korea in the Clinton administration demonstrated a good example to support Kydd’s argument. However, the trust-building between the United States and North Korea in the Bush administration that followed exhibited a different outcome. By comparing the signaling process between the Clinton administration and the Bush administration, I found that the postures held by different administrations have different impacts on a trust building process. Sending costly signals may only be efficient under certain circumstances, namely when the states are in a defensive posture. The offensive posture held by the administration may impede the trust-building process in several ways. The comparison between the two administrations thus shows some interesting implications, which can give us a further understanding of the relationship between the United States and North Korea and at the same time shed light on understanding trust and trust building in international relations.