Since Ballast Water Management Convention has been adopted, BWM Convention is effecting after 8SUPth/SUP Sep. 2017. Effecting conditions of this convention are to be satisfied for ratification by 30 countries, and representing 35% of world merchant shipping tonnage. Last Finland has ratified this convention on 8SUPth/SUP Sep. 2017. In the case of new ships, BWM convention is to be applied to new ships on or after the date of construction as of 8th Sep. 2017, and these vessels are to be equipped with Ballast Water Treatment System in progress. On the other hand, existing vessels inspected for IOPP between 8th Sep. 2014 and 7SUPth/SUP Sep. 2017, for vessels constructed on or before 8th Sep. 2017, are subject to vessel inspection at the time of the first IOPP periodic inspection after 8SUPth/SUP Sep. 2017 the D-2 standard through the installation of a Ballast Water Treatment System shall be observed. Until now, about 73 Ballast Water Treatment Systems certified by Administration have been be reported to IMO. Even, types of Ballast Water Treatment System are very various. So, the Risk of each Ballast Water Treatment System can be variously existed, and these Risks can be also effected to boarding crew safety and protection of ship`s property. Therefore, we have evaluated a Risk Assessment for an existing vessel retrofitting an Ultra Violet Type BWTS which is generally produced in worldwide. And we explained the detail procedure of selecting a object vessel. In conclusion, bulk carrier has been selected, because the sort of this vessel mostly charged in the world. In addition, DWT 175K size is selected. Risk Assessment is to study by using a HAZID and HAZOP method, evaluation method is referred to IMO Document