Objective: Orally administered vitamin B12 treatment is a more suitable option for children to adapt to the treatment. In this study, it was aimed to retrospectively evaluate the effectiveness of oral vitamin B 12 treatment [cyanocobalamin (Dodex ampoule) ® 1,000 μg/amp, orally on an empty stomach, 1,000 μg every day for the first week, 1,000 μg every other day for the next week, then 1,000 μg twice a week for two weeks, then 1,000 μg once a week for 4 months oral B12 maintenance therapy] conducted on children aged 6-18 years with nu- tritional vitamin B12 deficiency. Material and Methods: The files of 6-18 age group patients who were followed up and treated with the di- agnosis of vitamin B 12 deficiency at Karadeniz Technical University Faculty of Medicine, Department of Pediatrics, Division of Pediatric Hematology and Oncology between January 2010 and January 2018 were analyzed retrospectively. Results: The most common complaint was: headache (28.8%). Patients were diagnosed with high red cell dis- tribution width in 24.1%, anemia in 18.8%, lymphopenia in 7.5%, macrocytosis in 3.1%, neutropenia in 2.8% and thrombocytopenia in 2.7%. Homocysteine elevation was found in 30.1% and urinary methyl- malonic acid elevation in 15.6% of the patients. Serum vitamin B12 lev- els which are evaluated after treatment and hemoglobin values increased. Although these values were found to be significantly higher (p<0.05) compared to the values before the treatment, the mean serum B12 levels were 191.4 pg/mL (<200 pg/mL). In maintenance treatment, the success rate of 1,000 mcg cyanocobalamin treatment given for 4 months, once-a-week was 27.7%. Conclusion: Although oral treatment is effective, it has been concluded that a 4-month, once-a-week main- tenance treatment is not sufficient in 6-18 age group patients. Therefore, we believe that it will be more appropriate to offer an oral vitamin B12 maintenance treatment more frequently than once a week.