The precautionary principle has been implemented in many fields including environment protection, biological diversity, and climate change. In the field of international nuclear safety regulation, the implementation of this principle is in an ongoing process. Since Japan declared to discharge Fukushima nuclear waste water into the ocean, the precautionary principle was put on the stage, and some debates are invoked on it. As is observed by this article, the precautionary principle has not been effectively implemented in nuclear safety regulation, specifically in nuclear safety law making, law enforcement, and judicial application. The reasons can be found from two main challenges: indeterminacy of perceived risk level required to justify precautionary action and hard balance of national interest and community interest in nuclear safety. In a long-term perspective, the framework of international nuclear safety regulation has to respond to these challenges, both by clarifying the precautionary principle in legal binding nuclear safety documents and moving towards a more transparent, fair, and effective enforcement regime in order to promote safer, more sustainable, and efficient civilian nuclear utilization around the world.