In moving-ship type CO 2 ocean sequestration, liquid CO 2 is discharged into a domain in a water column. Since the maximum CO 2 concentration that is reached depends on the horizontal shape of the water column and the depths of release, it is very important to optimize these parameters for each injection site in order to minimize the biological impact. We conducted numerical experiments using an offline Oceanic General Circulation Model with a horizontal resolution of 0.1 degree 9 0.1 degree. Experiments using a different horizontal site shape show that a site elongated in the meridional direction is effective to reduce the CO 2 concentration. This is because CO 2 has a tendency to be transported in a zonal direction. Optimization of the vertical distribution of CO 2 injections is inherently determined by the balance of the following two factors; (1) dilution effect by eddy activity which decreases with depth, and the (2) predicted no effect concentration (PNEC), a criterion concentration causing no effect on biota, which increases with depth. Based on superposition of simulated CO 2 concentration, we determined the optimized vertical distribution of CO 2 injection which keeps the ratio of a simulated maximum CO 2 concentration to PNEC constant.