Aims: Fatigue is a frequent symptom in hemodialysis patients that impairs the quality of life and like pain and fever; it is an important biological warning for human health. So far, despite the many interventions to reduce fatigue in hemodialysis patients, still this phenomenon is reported with high prevalence. The aim of this study was to determine the effect of vitamin B12 supplementation on fatigue in patients undergoing hemodialysis. Methods: It was a semi experimental study with the method of before-after, which has been done on 86 hemodialysis patients with a Purposive sampling method within three months in 2011 in one and two chosen hospitals. The intervention was an intravenous injection of 100 mcg/ml of vitamin B12 after dialysis process which has been done three times a week, within two months and by the end of the intervention in order to assess sustainability of the effect of vitamin B12 there was no interventions for one month. Fatigue measurement questionnaire was completed by participants and it was analyzed by using SPSS18 software and descriptive statistic tests, paired t-test and analysisvariance of the repetitive measures. Results: Fatigue after a month of interfering (p=0.001) and with further reduction after two months (p=0.0001) and one month after the end of intervention (p=0.021) has been significantly reduced. Conclusions: The results indicate the effectiveness of vitamin B12 on the reduction of fatigue in hemodialysis patients. Thus intravenous injection of 100 mcg/ml of Vitamin B12 weekly after dialysis is recommended for hemodialysis patients. Please cite this paper as: Tayyebi A, Savari S, Nehrir B, Rahimi A, Eynollahi B. The effect of Vitamin B12 s supplementions on fatigue in hemodialysis patients. Iran J Crit Care Nurs 2013;6(1):39-48.