Objective To explore the entrapment to the deep branch of the ulnar nerve at the wrist, to assist the surgeon in diagnosis and treatment of Guyon canal syndrome. Methods Twenty fresh upper limbs were dissected on loupe(×5), to record the relationship between the ulnar nerve and the hypothenar muscles, the course of the ulnar nerve at the wrist. Results The deep branchs of the ulnar nerve go through an intermuscular space after runing out of the Guyon canal. The intermuscular space consisits of the superficial and deep head of the opponens digiti minimi and the hook of the hamate, which has one entrance and one exit. We named the intermuscular space as the hiatus of opponens digiti minimi, and named the proximal edge of the superficial head of the opponens digiti minimi as the fascial arch of the opponens digiti minimi.Conclusion We found that the fascial arch of the opponens digiti minimi can compress the deep branch of the ulnar nerve causing motor deficit of the intrinsic muscles of hands.