Objective: Many drug or non-drug therapies are reported to be effective in reducing the severity of attention deficit/hyperactivity disorder (ADHD). The aim of this study was to assess the effect of color and pattern on the severity of ADHD in young ADHD patients. Method: We randomly assigned 30 children into two groups and had them participate in 15-minute block play sessions once in a week. The experimental group used blocks with green (3G, Munsell color system) circle patterns on them, while the control group played with plain white blocks. We also provided a set of playing cards with the corresponding patterns. This trial continued for 4 weeks and 29 children (96.7%) completed. The primary outcome measurement was the ADHD rating scale-IV and was evaluated on every visit. Results: After the trial, ADHD score decreased in both groups but not significantly ($34.4{pm}8.0$ to $30.0{pm}4.3$; $34.0{pm}6.0$ to $30.3{pm}3.2$). However, there were significant differences in 4 items of ADHD rating scale between the two groups. These consisted of 3 inattention (Q3, Q7, Q15) and 1 hyperactivity-impulsivity (Q12) items and the reduction between the first and final visits was significant (p=0.0240, p=0.0498, p=0.0237 and p=0.0012 respectively). Conclusion: Repeated exposure to a green circle pattern might not influence the total score of ADHD rating scale but reduced 3 inattention items and 1 hyperactivity-impulsivity significantly in children with ADHD. Proper visual stimuli are partially helpful for pediatric ADHD.