This paper is concerned with reliability assessment of ship transverse structures, which are modeled as relatively rigid frame and subjected to quasi-static extreme loads, based on plastic collapse analysis. Plastic collapse is evaluated by using a linearized plasticity condition of the section under the combined effect of bending moment, shearing force and axial force to generate the safety margins, using a matrix method. Probabilistically dominant collapse modes are selected by applying the so-called branch-and-bound method. The above method is successfully applied to the transverse structures of large and medium size tankers and an ore carrier under some notional load conditions. Through the numerical examples, probabilistic properties of plastic collapse of ship transverse frame structures are investigated.