This paper is intended to discuss the controversial issue of the principles of change circumstances under the legal system of international commercial transactions. The principles of change circumstances, so called clausula rebus sic stantibus is the legal doctrine allowing for treaties to become inapplicable because of a fundamental change of circumstances. It is essentially an "escape clause" that makes an exception to the general rule of pacta sunt servanda (promises must be kept). The practical needs of international transactions differ from the established concepts of national contracts law. The purpose of this paper is to analyze the legal system and theories under the regimes of international commercial transactions such as the CISG, the PICC, and the PECL. Clausula rebus sic stantibus does not apply if the parties to a treaty had contemplated for the occurrence of the changed circumstance. It only relates to the changed circumstances that were never contemplated by the parties. This paper has shown that the hardship provisions in the CISG, PICC, PECL has similarities to each a validity defense and an excuse defense. it was provisions that CISG governs this issue in Article 79, PICC Article 6.2.1, 6.2.2, 6.2.3(in addition to Article 7.1.7), PECL Article 6.111(in addition to 8.108). It is time when we should reconsider its legal system with great interest in order to harmonize with the international standpoint. It will be the turning point of our viepoint under the international commercial transactions.