Geometrical sequence and series are mathematic topics that should be mastered by students, but some students encounter obstacles in learning these topics. This research aims to identify the learning obstacle by students in solving geometrical sequence and series problems. The research question was what kind of learning obstacles students face in the topic of geometrical sequence and series. The research is qualitative method with a case study research design. The participants were students of senior high school in Cirebon. Data collection was done by conducting tests and interviews. The test method was to identify the learning obstacles faced by the students and interviews as supporting data. The results show that students meet learning obstacles in solving geometrical sequence and series problems, namely ontogenical obstacle, didactical obstacle, and epistemological obstacles. Ontogenical obstacles were experienced by those students who do not understand the concept of geometrical sequence material, and that the answer written was wrong. The didactical obstacle is experienced by those students who find it easier to do it manually compared to using formulas in solving geometrical sequences and series problems. An epistemological obstacle is experienced by those students who are not able to write down the correct answers because the questions are different from habits.
Keywords: geometrical sequence and series, learning obstacle, senior high school