In this paper, an equivalence relation on the class of uninorms induced by the U-partial order is discussed. Defining the set of all incomparable elements w.r.t. the U-partial order, this set is deeply investigated and some relations with the sets of all incomparable elements w.r.t. the orders induced by the corresponding underlying t-norm and t-conorm are presented. Also, the set of all incomparable elements with a fixed element w.r.t. the U-partial order is defined and studied in detail. (C) 2017 Elsevier Inc. All rights reserved.