Controlled experiment, an approach that has been adopted from research methods in psychology, is now widely used in HCI. Design an effective controlled experiment is not necessarily easy, even for experienced researchers. Existing controlled experiment designing tools focused more on the process of designing, but they are often not intuitive and guided enough for less experienced users to use. In this demo paper, we introduce NexP (Next Experiment Tool), a web-based open-source tool for designing controlled experiments. NexP introduces a 5-step approach to guide users through the experimental design process, and helped them to better understand the experimental design process, making it both a useful and educational tool.