This study examined the utility of preliminary magnetic resonance imaging (MRI) measurements in the ultrasound-guided L4 selective nerve root block. Materials and Methods: As a retrospective study, 71 patients, who met the criteria for outpatient visits from March 2016 to December 2017, were included. From March 2016 to February 2017, 31 patients who underwent an L4 nerve root block without MRI were classified as group A, and 40 patients who underwent an L4 nerve root block through MRI measurements from March 2017 to December 2017 were classified as group B. Group A was injected under ultrasound-guidance through the pararadicular approach without a pre-interventional MRI evaluation, and group B was injected under ultrasound-guidance according to the preliminary MRI measurements. The results were assessed using the numeric rating scale scores before, three hours, and two, six, and 12 weeks after the procedure. Results: At three hours after the procedure, the proportion of patients better than good results were 51.6% in group A and 67.5% in group B. At two weeks after the procedure, the proportion of patients with better than good results were 48.4% and 70.0% in groups A and B, respectively; 58.1% and 62.5% of patient of groups A and B, respectively, showed better than good results after six weeks. In 12 weeks after the procedure, the results of group A and B were 67.7% and 62.5%, respectively. At three hours and two weeks after the procedure, group B showed significant symptom improvement than group A (p<0.05). The procedures were repeated 2.8 and 1.7 times in groups A and B, respectively, between two and six weeks for satisfactory pain relief (p<0.05). Conclusion:A pre-interventional MRI evaluation might improve pain relief within the initial two weeks after ultrasound-guided L4 selective nerve root block by improving the success rate of the procedure.