Abstract : The ability of conjecturing thinking skill is very important in the industrial era 4.0. Conjecturing thinking skills are the skills to make strong guesses or predictions on problem solving. This skill is also strongly related to the skill of generalizing from a specific truth to a broad truth. Therefore in this study, the Research-Based Learning model which is integrated with the STEM approach (Science, Technology, Engineering, Mathematics) will be applied to solve the STEM problem of paving block decorations. Students are introduced to the application in the form of local antimagic coloring techniques. The next step is how to bring STEM problem solving in learning class, then the developed RBL and STEM tools in learning to improve students’ conjecturing thinking skill. Base on this study, it shows that the problem solving elaborated with four elements of STEM is considered to be a breakthrough in applying RBL-STEM in the classroom to solve the paving block decoration problem.