Entrapment of the extensor pollicis longus tendon is reported rarely on Smith's fractures in children. In our case, a 15 year old boy with Smith's fracture received treatment of closed reduction at another hospital. When he visited our hospital, a wide gap at the fracture site was detected on radiograph and the thumb movement was limited. We have doubt the entrapment of the soft tissue, especially the tendon. We decided on open reduction. In the operation field, entrapment of the extensor pollicis longus tendon at the gap of the fracture site was found through dorsal approach. In addition, fracture treatment with K-wire fixation after reduction of extensonr pollicis longus tendon reduction was done. Therefore, we report this case with a review of the literatures.