Ulnar nerve involvement is the second most common local mononeuropathy. The ulnar nerve consists of motor and sensory fibers that arise in C₈-T₁ roots and extends to the lower trunk and medial cord of brachial plexus. Manifestations of this nerve involvement range from elbow pain and intermittent hand paresthesia to marked sensory loss, wasting and weakness  of hand muscles and a claw hand. Ulnar nerve lesion in the elbow region commonly follows surgery in which general anesthesia is used (particularly in patients with coronary artery bypass graft surgery). The patient presented in this article was a 56-year-old male who was involved in left ulnar nerve lesion after undergoing coronary artery bypass graft. In electrodiagnostic study we noticed diffused demyelination lesion of the ulnar nerve with dominancy in elbow region along with secondary axonal degeneration. Therefore, suitable positioning of elbow is recommended to avoid the  nervous complications of these operations.