In the mooring operation of semi-submersible offshore units, mooring anchors and cnains are deployed from units by anchor handling boats. In this case, there are two types of deployment method. In one method, all chains are at first dropped onto see bed, then deployed. In the other method, both dropping and deploying operations are performed simultaneously. To make it clear which deployment method is more efficient, it becomes necessary to investigate behaviors of chains and pull-loads of boats under deployment. In this paper, a series of analyses on deployment operations are carried out using a newly developed simulation model where equilibrium states of chains are evaluated based on catenary theory and dropping motions of chains can be considered. According to the results, optimum deployment method and control pattern for dropping chains are discussed.