Since investment arbitration is in nature dealing with dual cause of actions as both treaty claim and contract claim. The issues of its application of laws are reflected with certain diversity and complexity. This article mainly discusses concrete issues relating to variety of governing laws applying to procedure matters and substantive disputes in international investment arbitration, which focuses on demonstrating the principle, scope and effect of the application of international law and domestic law, and the relation between application of the two legal systems. Meanwhile, applicable law of state contract and construction of BIT applying are also discussed.