The aim of this study is to evaluate the effects of nutritional supplement co-enzyme Q10 (CoQ10) on methylphenidate-treated ADHD children in a randomized, double-blinded, placebo-controlled prospective study. All eligible patients will undergo randomization and divided into 2 groups: a CoQ10-enriched snack and a placebo snack group. According to power calculation, total of 60 subjects are expected to participate in the study. After the screening of eligibility (up to 14 days), the study is divided into three phases: pre-treatment (first assessment) phase (up to 14 days), treatment phase 8 weeks of treatment, and post-treatment phase. Screening: each participant will undergo screening for protocol eligibility within 14 days (two weeks) of recruitment. Subjects who meet all the inclusion criteria and signed an approved informed consent (both parents and the child) will be enrolled.