The vulnerability of a warship is defined as its inability to withstand a man-made hostile environment, and can be estimated by the conditional probability of being killed by a hit. We describe ship vulnerability given a penetration hit, and propose a vulnerability assessment procedure that incorporates a vulnerable area approach to naval ship survivability. Measures of vulnerability indicate the killability of critical components with respect to the effectiveness of hostile weapons. In the proposed methodology, a warship is considered to be an assembly of critical components representing an entire vulnerability. We evaluated the vulnerability of a warship subjected to penetration effects using hypothetical models because of the paucity of available data and information, and the effectiveness of such assessment methods during initial warship design. The proposed approach introduces critical component redundancy and overlap, the effects of single hits and multiple hits, and attempts to directly apply the vulnerability assessment technique to vulnerability reduction. The kill tree method, Markov chain method, and Poisson method are applied to multiple hits on critical components. Examples show that the proposed method can provide the vulnerability parameters of a warship under the threat of being hit by a vulnerable area approach, thereby enabling an assessment of vulnerability.