A distributed dynamic micro-mobility management scheme is proposed to make up the deficiency of not supporting well for highly Mobile Host (MH) in Mobile IP. The scheme places a few of Regional Mobility Agents (RMA) to distributedly manage hosts’ mobility in region. An algorithm is presented for MH to decide its RMA and regional size in terms of its mobility characteristics and some informed network parameters, which results in minimum signaling cost and packet delivery cost, but does not impose any restrictions on network’s topology and RMAs’ locations. Analysis shows when MH’s average packet arrival rate increases, the regional size decreases while the total cost increases; when MH’s average residence time in Access Router increases, both the regional size and total cost decrease. Finally, a performance comparison demonstrates that the total costs produced in Hierarchical Mobile IPv6 with several distinct regional sizes are all higher than the possible highest total cost produced in the scheme.