This study was performed in order to investigate the relationship between attention deficit hyperactivity disorder (ADHD), dietary habit and caffeine intake in upper-grade elementary school children. The total number of the study subjects was 237 students (111 boys and 126 girls), where 30 students (12.7%) were diagnosed as ADHD. The dietary habit score of the ADHD group was significantly lower than that of the normal group. In particular, the ADHD group had lower dietary scores in consuming daily breakfast, diverse foods, fruit and milk than those in the normal group. Meanwhile, the daily intake frequency of instant noodle (ramyeon) was significantly higher in the ADHD group than that in the normal group. The mean caffeine intake of the students was 42.95 mg and the proportion of students consuming more than the ADI (acceptable daily intake) was 11.8%. The caffeine intake of ADHD group (63.63 mg) tended to be higher than that of the normal group (39.95 mg); however, it was not significantly different. The ADHD score of the students was negatively related with the dietary habit score (r = -0.279, p < 0.01) but positively related with caffeine intake (r = 0.164, p < 0.05). The dietary habit score had a negative relationship with caffeine intake (r = -0.180, p < 0.01) and a positive relationship with height (r = 0.195, p < 0.01). Caffeine intake had a negative relationship with the height of the students (r = -0.171, p < 0.05). In conclusion, ADHD in children was related to poor dietary habit and high caffeine intake.