Courts around the world have been confronted with bewilderingly complex challenges in protecting the public interest through copyright law. This article proposes a public interest principle that would guide courts to settle fair use cases with better-informed decisions. I argue that the proposed principle would legally upgrade fair use from serving as an engine of free expression to serving as an engine of public interest protection. Based on comparative study of the conflicting rulings handed down by the U.S. and Chinese courts on Google Library, the article first considers the necessity of adopting the public interest principle in guiding the judicial settlement of fair use cases substantively and procedurally. The article then canvasses the two substantive legal standards to be embodied in the public interest principle. First, the principle would create a public interest use standard for courts to utilize in weighing the first fair use factor without applying the dichotomy of transformative and non-transformative use. At the same time, it would also require courts to employ the significant market harm standard when considering the fourth fair use factor. Second, the public interest principle would also modify the procedural rules concerning the assignment of burden of proof in fair use cases. It would place only the burden of proving a public interest use under the first factor on the user of a work who is the defendant in the judicial proceedings at hand.