Smartphones are playing an increasingly important role in helping people share information and communicate with each other, as evidenced by an increasing number of applications (or apps), including news, search engine, mailbox, and social networking apps. The more heavily people rely on their phones, the more they begin to take the term “user experience” into consideration when buying a smartphone. Obviously, the key to providing a good user experience is to minimize the response time of each graphical user interface (GUI) operation, such as a touch, slide, or flick. Thus, it is crucial for smartphone manufacturers and mobile app developers to measure the response times to enhance the performance of the device or application. In this article, the authors present the SMArtphone Response Time measuring platform (SMART), a novel and low-cost platform for measuring the response time of a smartphone. SMART consists of a normal 30-fps (frame per second) webcam and a mirror. The proposed platform first simultaneously records the changes on a smartphone screen and the GUI operation shown in the mirror. It then adopts image processing techniques to analyze the latency of the GUI operation. The authors’ validation results show that the measurement error of SMART is less than 1/30 s, which means that SMART is a practical solution for measuring the latency of GUI operations. In addition, they adopted SMART for the measurements and analysis of six different commercial smartphones to further demonstrate its applicability.