Spontaneous osteonecrosis of the tarsal navicular, called the Mueller-Weiss syndrome, is an uncommon disease. Patients who are resistant to conservative treatment require operative treatment. However, there is no established operative treatment. Two cases of spontaneous osteonecrosis of the tarsal navicular with double (talonavicular and naviculocuneiform joints) arthrodeses with different locking plates are presented. Removal of necrotic areas from the tarsal navicular and replacement with autologous bone graft procured from the iliac crest followed by arthrodesis using a locking plate were performed. Case 1 was fixed with an LCP Distal Radius Plate (SYNTHES) and 6 2.4 mm locking screws. Case 2 was fixed with a Cervical Spine Locking Plate Variable Angle (SYNTHES) and 4 4.0 mm locking screws. Case 2 achieved solid fusion of the talonavicular-cuneiform joints, but case 1 resulted in nonunion of the talonavicular joint. This difference in internal fixation strength might have caused the difference in the results. Performance of double arthrodeses from the medial aspect using a locking plate is a reasonable operative procedure to treat spontaneous osteonecrosis of the tarsal navicular. Strong primary fixation using a thick plate with large-thread screws was important to obtain joint fusions.