In this article, we report a case with spontaneous rupture of extensor pollicis longus tendon due to calcified extensor carpi radialis tendon group, which has not been published in the literature previously. Successful thumb extension was achieved in a 68-year-old male patient with the transfer of the extensor indicis proprius tendon at two months postoperatively. It appears that calcified extensor carpi radialis tendon group constitutes high risk for a possible attritional rupture of the extensor pollicis longus tendon.