This study aimed to investigate student understanding of graphs in simple harmonic motions using eye-tracking technique and an interview. Twenty-three university students participated in answering 11 multiple-choice questions, adopted from Somroob & Wattanakasiwich (2017). Students’ responses and visual attention were recorded by an eye tracker and they were interviewed to provide reasoning on their answers. As results, students had similar misconceptions found in previous studies. The heat map from eye-tracking technique helps to identify and categorized student abilities in interpreting graphs. Their interpretations could be categorized into novice, mixed-model and experienced state.