Introduction: Humerus shaft fractures may be treated conservatively or surgically. In 2.5% to 13% of cases, nonunion is observed, and it leads to severe pain and morbidity. Plate osteosynthesis has become popular in the treatment of nonunion of the humeral shaft. In this study, we compared the clinical outcomes of patients with humerus shaft nonunion whom we treated with single- or double-plate fixation. Materials and Methods: Fifty-three patients diagnosed with aseptic humeral shaft nonunion and treated with plate fixation were included in the study. Patients were evaluated according to the number of plates (single vs. double plates). The two groups were subjected to statistical evaluation according to their clinical and radiographical results. Results: The average age of the patients was 53 years (range: 1-86); 28 (52.8%) were female and 25 (47.2%) were male. The union rate was 90.32% for single plate and 90.91% for double plate fixation. There was no statistically significant difference between single and double plates in the clinical and radiographical results (union time, union rate, Q-DASH score) (p > 0.05). There was a significant correlation between age and union times / Q-DASH scores according to the Spearman correlation test (p < 0.05). Transient radial nerve neuropraxia developed in 2 patients and 1 patient suffered from an infection that was treated with debridement and antibiotic therapy. Conclusion: In our study, similar good results were obtained with single and double plates. In treatment of humeral shaft nonunions, a second plate is not needed if enough stability is provided with single plate fixation.