The variety of fiduciary legal relations in the civil law of Ukraine requires the study of their individual elements, in particular, the fiduciary duty. Therefore the purpose of this article is to clarify the legal nature of the fiduciary duty, in order to avoid legal uncertainty, which leads to difficulties in law enforcement and, as a result, ineffective legal protection of violated rights of a person due to non-fulfillment or improper fulfillment of such an obligation in relation to her.Analyzed such concepts as "fides", "fiducia", duty in civil law. It has been established that the first, respectively, in Roman law had several meanings, but the main thing is the trust of the participants in civil relations to each other. The second have to understood as the proper behavior of the subject of civil relations, the content and model of which are determined by the requirements of the rule of law, the will or persons authorized by the transaction or other legal facts.It was found that the fiduciary duty is the proper behavior of the subject of a trust relationship, due to the conclusion of certain agreements (for example, commissions, property management, joint activities, the provision of lawyer services, etc.), or the occurrence of legal facts (election of a body or person of a legal entity, who (who) has the right to act on her behalf, the establishment of guardianship or trusteeship, the death of an individual, etc.).