Catastrophes in petrochemical plants dealing with a large amount of flammable materials are most liable to be caused due to destruction or loss of functions of equipment or installations accompanied by great earthquake. In this paper, two safety assessment methods to prevent such disasters are presented. One is to determine, by keeping potential danger of fires and disastrous explosions of facilities where flammable materials are dealt with in mind, the degree of the anti-seismic design of facility in accordance with the extent of hypothetical disaster (Importance Classification). Another is to make comparison with the societal acceptance level by keeping a disaster scenario at the time when a plant encounters a great earthquake in mind and by investigating the events accompanying the scenario with the attachment of a probability. Furthermore, by inquiring if there is any influence outside the premises even though there occur fires and disastrous explosions at the level, the safety of the facilities is evaluated.