Application Programming Interface (API) plays a vital role in Cloud computing. Improving API usability will help application programmers select and adapt the most suitable APIs for their specific needs. By adopting a general framework for usability measurement and analyzing questions and answers on Stack Overflow for YouTube APIs, we define three direct usability metrics: “team response time" to quantify learnability, “net upvote" to measure user satisfaction, and “downvote" to reflect usability problems. In addition, we identify six environmental factors that influence API usability by analyzing the question tags: language, library, browser, operating system, integrated development environment, and device. Our measurement and analysis provide an objective assessment of API usability and its environmental influencing factors.