Root decomposition is of great significance for carbon sequestration and long-term soil nutrients availability, but the long-term decomposition patterns of different root diameters were poorly understood. In this paper, we compared the seven-year decomposition processes of different root diameters for three temperate broadleaf tree species to provide data supporting for ecological process model. We conducted a seven-year field decomposition experiment for five root diameter classes (< 1 mm, 1-2 mm, 2-5 mm, 5-10 mm, and 10-20 mm) of Betula platyphylla (BP), Ulmus davidiana var. japonica (UJ), and Fraxinus mandshurica (FM) with a litter-bag method at the Maoershan site. Repeated measures analysis of variance showed that time, tree species, the interaction of diameter and tree species, and the interaction of tree species and time significantly affected the root mass remaining. The root mass remaining exponentially declined with time; During the seven years, the roots experienced two periods, i.e., a rapid loss in the early stage and a slow loss in the later stage; At the end of the experiment, the roots remained a considerable proportion of its initial quality (24%-56%). The decomposition constant of roots with different diameters for each tree species was estimated with the Olson exponential decay model. There was a significant linear positive correlation between the decomposition constant and root diameter for BP, while a significant positive logarithmic correlation for FM, and no significant relationship for UJ. The short-term decomposition constants of the three tree species overestimated root decomposition rate, and they could not fully represent the intra-species and inter-species differences in the long-term decomposition constants. The results provide data support for understanding the varying pattern of long-term root decomposition rate with root diameter.