Background & Aims: At present due to brain dead patients’ families refuse to organ donation, the number of available organs is less than the requesting cases. The aim of this comparative descriptive study was to compare opinions of donor and nondonor families about relevant factors in their decision making. Methods: participants included 175 members of 71 families (141 members of 56 donor families and 34 members of 16 nondonor families). Data were collected by a questionnaire after validity and reliability confirmation. Results: There was statistically significant difference between two groups in initially favorable reaction to the request, presence of the effective individuals on decision making in first cession of the request, awareness of the deceased beliefs about donation, and awareness of the conditions of patients with organ failure (P<0.0001). Regression analysis revealed that two first factors are the decision predictors (P=0.001). In regard to effective factors on decision, religious beliefs, trust, caring, decision doubt and conflict and transplant beliefs were significantly higher in donor group (P=0.0001). Regression analysis showed religious beliefs and transplant beliefs as predictors of the consent to donation (P=0.0001). Conclusion: Consent to organ donation is affected by two groups of factors and can be increased by providing public education and adequate conditions at the time of organ request based on the needs and critical conditions of relatives. This requires more attention of the Organ Procurement Organizations and also employing skilled and experienced individuals in organ request process.