In this paper environmental impacts of deep ocean mining are briefly reviewed, with emphasis on the effects on oceanic ecosystem. They include effects on benthos, planktons, food chain and so on. The authors point out that the advantages of deep ocean mining to the ecosystem should be further studied and that the monitoring data on the ocean mining test are not the same as those to be obtained during commercial mining in the future. Therefore, attention should be paid to this point when studying the potential environmental impact using the monitoring data.