The preliminary security assessment is an information security process to analyze security weaknesses before beginning of services. Discovering security weakness through preliminary security assessment is highly required because it costs much when security incident occur in the middle of service operation. However, this assessment is not widely spread in the online game service domain yet. In this paper, we summarize the security risk existed in the online game service, and we classify the security requirements related to the each risk. Also, through the case study, we evaluated the effectiveness of preliminary security assessment in this domain. In addition, we suggest checklists that should be reviewed once in game-client side, network-side and game-server side for the purpose of security enhancement.