Multicriteria decision-making problems with initial information about the fuzzy preference relations-type between the couples of alternatives by each criterion are considered. This paper deals with the union of these relations, such that the aggregated relation is a fuzzy preference relation as well. Two ways to get aggregated fuzzy relations are presented. The main contribution of this research consists in establishing the connection between the union relation and the private relations properties. These properties, required for the various relations according to different criteria in order to get a union relation helping the decision of ranking or choice problems, are theoretically proved.