Abstract: User association problems in millimeter- wave small cell networks are more complicated due to its characteristic of directional LOS transmission and sensitivity to obstacles. A distributed user association algorithm based on fairness criterion in millimeter-wave microcell network is proposed. Firstly, the instability of millimeter-wave channel and user switching cost are modeled. Then the user association algorithm based on fairness criterion is formulated and proved to be a convex problem. The Lagrangian duality theory is used to solve the original problem. The analysis and simulation results show that: (1) When the handover cost is low or zero, user equipment (UE) should be only associated with one SBS; (2) When the cell handover cost is high, UE should be associated with multiple SBSs simultaneously; (3) Compared with other algorithms, the proposed algorithm has a significant gain in terms of throughput.