International surveys, such as TIMSS and PISA, frequently put Indonesia in the low ranks. It is an indication that the higher-order thinking skills (HOTS) of students in Indonesia are still low. This research aims to analyze students' difficulties in solving problems that measure HOTS. This is a case study research with a qualitative approach. Participants studied were 93 high school students in grade XI. Data were collected using test instruments that measure HOTS, which was developed based on the standard contents of high school mathematics. The difficulties were analyzed descriptively by observing students' errors in answering HOTS test items. Students' errors were classified based on Newman's Error Procedure (NEP). The result shows that around 8.33% of the students had difficulties in comprehension, 15.59% in transformation, 32.53% in process skills, and 1.34 % in encoding.