The goal of this study is to develop and examine the effectiveness of an Ecosystemic Play Therapy (EPT) Program for suspected attention-deficit/hyperactivity disorder (ADHD) pre-school children. The program was developed from a pilot experiment and then a randomized controlled trial was conducted to examine its effectiveness. 12 parent-child dyads were ran-domly assigned to the experimental group and the waitlist control group. The experimental group received the EPT Program for 6 weeks. Outcomes were measured with the Atten-tion-Deficit/Hyperactivity Disorder Test and the Conners’ Kiddie Continuous Performance Test II at baseline, after treatment, and at follow-up. For each child participant, a parent would complete the Ecological momentary assessment (EMA) protocol with the Activity Level Rating Scales for Parents. The percentage of attendance was 99%. Although the experimental group showed a little more severe inattention symptoms before the treatment, they showed significant improvement in ADHD symptoms and behavior problems, while the control group showed significant improvement in partial symptoms, “Omissions”, “Commissions”, “Per-severations” and “Hit Reaction Time”. Comparing the differences between the two groups, the experimental group showed significant improvement in attention and behavior after the treatment, and again significant improvement in hyperactive/impulsive symptoms and be-havior when reevaluated at the follow-up appointment. EMA results showed the behavior problems of the experimental group improved significantly in the first 3 weeks and continued to the follow-up. In conclusion, the EPT Program has positive effects on suspected ADHD preschool children. Research based on the results to discuss and make recommendations for practice and further studies.