Objective:A patient with L3-5 lumbar disc herniations and right spinal recess stenosis at L4-5 level associated with type IIB lumbosacral nerve anomaly(LSNA) at right side according to Neidre and Macnab classification system missed preoperatively and initially misdiagnosed as type IIA intraoperatively was reported. The reasons leading to missed diagnosis preoperatively and misdiagnoses intraoperatively were analyzed. Methods: A 62-year-old female was admitted to our hospital for intermittent radiating pain and claudication to the right lower extremity for about 20 years, radiating pain to the left lower extremity for 2 weeks. Physical examination found the left L5 nerve and right L4 and L5 nerves were impinged. CT and MR scannings revealed L3-4 disc herniation at right side, L4-5 disc herniation at central and left side with severe lateral recess stenosis at right L4-5 level. Image examinations failed to find a nerve root anomaly preoperatively. Following failed conservative treatment, the patient accepted decompression operation initially at left side. Left L5 nerve compressed by the herniated L4-5 disc was found firstly, which was liberated by discectomy. When doing decompression exploration at right side, it was found that two lumbar nerve roots exited L4-5 intervertebral foramen while the upper one was compressed by the herniated L3-4 disc which was liberated by discectomy and the lower one was free of compression. With exposing the right side and partly unroofing of stenosed right L4-5 lateral recess , no typical L5 nerve root was found passing over the posterior surface of the herniated L4-5 disc except that there was a slim membrane-like structure covering it. The patient was initially considered as type IIA LSNA skeptical according to Neidre and Macnad Classification. But detailed observation of the axial CT and MR sequences at L4-5 lateral recess space and sagittal CT and MR sequences at L5S1 intervertebral foramen found there was nerve-like structure, which was further confirmed to be the L5 nerve root by total unroofing of stenosed lateral recess. The membrane-like structure covering the L4-5 disc was confirmed to be the flattened right L5 nerve root compressed by herniated L4-5 disc and stenosed lateral recess, which emerged from the dura’s ventral-lateral side just above the L4-5 disc, the L5 nerve was stripped and pushed medially from its lateral boundary meticulously, L4-5 discectomy was done at right side. Intervertebral cages were inserted after L3-4 and L4-5 disc spaces preparation, following pedicle screw system fusion. Results: No nerve damage occurred intraoperatively. The patient was free of radiating pain to the left lower extremity the day after operation, two weeks later the radiating pain to the right lower extremity alleviated after temporary aggravating. Conclusion: type II lumbar nerve anomalies were not easily to be detected by CT or MR images preoperatively, that closer distance between the conjoined nerves existing the L4-5 foramen and careless observation was responsible for missed diagnosis preoperatively. That the flattened right L5 nerve covering the L4-5 disc was difficult to be recognized intraoperatively for losing its typical contour and color was responsible for misdiagnosing type-IIB anomaly as IIA LSNA.. The operators should be alert and observed carefully in case of misdiagnosing.