In order to assess risk as a basic step for securing safety, it requires to select risk factors and determine the frequency and the severity of the consequence of each risk factor. This research adopted common risk factors among well-known maritime risk assessment models, and proposed objective criteria to gauge the risk level of each risk factor. The starting points of risk evolution were chosen for criteria according to related studies and seafarers’ experience. The rate of risk appearance over the criteria is named as the incidence of risk factor. Therefore, the total risk level is expressed as the combination of incidence of each risk factor and severity. This quantitative method would be applied to measuring and comparing the risk level of target maritime zones, and it would also be useful to survey which risk factor be focused for reducing the total risk of a certain maritime zone.