Many students experience difficulty in solving function composition problems. The aims of this study were (1) to find out the difficulties experienced by students in solving function composition problems, and (2) to find out why students experienced difficulties in solving function composition problems. This research is qualitative research which involved six students in grade X. Data was collected by test and interview with the researcher as main instrument and essay test and interview guidelines as supporting instruments. Data was analyzed through three stages, namely reducing data, presenting data and drawing conclusions. Data validity through method triangulation. The results of this study are that students experience difficulties in understanding the concept and applying the principle. The causes of students having difficulty in solving function composition problems are lack of mastery of the concept and knowledge of function composition material, lack of accuracy in substituting x values, students are not used to problems that are different from the example problems, lack of students' confidence to ask the teacher, students lack motivation in learning mathematics and are not interested in paying attention to the lessons explained by the teacher, and students never repeat or re-study the material.