本文的研究問題為：在假設原創者與利用者於同一市場之前提下，著作權合理使用第四要素該如何分析？透過賽局模型的推演，本文推導出二步驟分析法。第一：市場分析。若原創者與利用者在同一市場內競爭，則進行下一步驟分析。第二：成本分析。複製使用成本所投入的創意和巧思是否比轉化使用所投入的創意和巧思來的少？若答案為是，則根據賽局模型的情境比較分析可推定利用者會持續選擇複製使用原創者的作品，並對原創者現有與潛在市場產生負面影響。若答案為否，則利用者有誘因轉去選擇轉化使用原創者的創作，此時根據上述的比較分析可進一步推斷，利用者持續地轉化使用不一定會對原創者現有與潛在市場產生負面的影響，端看利用者的轉化使用成本與原創者的原始創作成本之大小關係。

Fair use doctrine has generated various discussions. However, Federal courts and the academics have only focused on the analysis of the first factor. The fourth factor, the effect of the use upon the potential market for or value of the copyrighted work, has been neglected for a long time. The disregard of the fourth factor delves into our research question of this paper: how do we analyze the fourth factor of the fair use doctrine? To answer this question, we developed a game-theoretic model and formed a two-step framework to analyze the fourth factor. The first step is the market analysis in that we argue if the originator and the user are in the same market to compete, then we must move further to the next step of the analysis. If not, then how the user utilizes the works of the originator has no relation to the originator. The second step is the cost analysis in which we analyze whether the cost of reproductive use is lower than the cost of transformative use. If the answer is yes, the user would be assumed to have strong incentives to continuously copy the originator’s works and would negatively impact the originator’s market. If the answer is no, the user would have incentives to shift to use the originator’s works in the transformative way. Furthermore, the impacts of transformative use to the originator depends on whether cost of transformative use could be continuously lower than the original cost of creation. If the answer is yes, the user would be assumed to impact the originator’s market negatively. If the answer is no, the transformative use by the user would be assumed to have little impact to the originator’s market.