ADHD comorbidity has been associated with delayed diagnosis of ASD, but no study has investigated this association in an Asian country. Children with ASD were included and divided into three groups: ADHD before ASD, ADHD same/after ASD, and ASD only. Timing of ASD and ADHD diagnoses were assessed. The logistic regression model was performed to investigate the likelihood of being diagnosed with ASD after 6 years of age between three groups. ADHD before ASD (OR 10.93) group was more likely to being diagnosed with ASD after 6 years of age compared with ADHD same/after ASD (OR: 1.37) and ASD only groups. ADHD comorbidity would delay the diagnosis of ASD in the general clinical settings in Taiwan.