This paper is concerned with reliability assessment of ship main structures, which are modelled as spatial frame structures and subjected to quasi-static extreme loads, based on ultimate strength analysis. Ultimate strength of the structure is evaluated by using a linearized plasticity condition of the section of the structural element under the combined effect of internal forces and moments to generate the safety margins of structural failure modes, using a matrix method and plastic node method. Probabilistically dominant collapse modes are selected by applying the so-called branch-andbound method combined with heuristic operations. The above methods are successfully applied to main structures of a large tanker (240, 000DWT) under various notional loading conditions. Probabilistic properties of ultimate strength of ship main structures are investigated through the numerical examples and comparison with previous results for plane frame models.