Risk assessment and benefit estimation of TBT compounds used as anti-fouling paint to prevent hull and culturing net being protective from propagation of sessile organisms was performed in perspevtive of regional basin, Osaka Bay in Japan. It was based on environmental fate simulated by using Basin-wide ecological model which was validated by comparing of regional observed data. Predicted concentrations in each compartment were respectively 1.9-466.8ppt in water, 3.0-155.0ppb in sediment and 0.02-0.93ppm in fish, which were almost same order of regional observed data. Risk of TBT was evaluated very high on aquatic life in some places which was estimated by dividing predicted concentration in Benthos (very sensitive species) with MATC. While it was low on human being to extent negligible which was evaluated by dividing predicted intake with ADI. Benefit of TBT was estimated as relative benefit to copper paint by using the indicators of energy saving and dry docking period of ship.