The ultimate strength is one of very important criteria for safety assessment of offshore structures. In this report, the ultimate strength of jack-up rigs is evaluated by the “Idealized Structural Unit Method”. Two loading conditions are taken into account, i. e. survival and punch-through conditions, which may lead to total collapse of the rigs and, then, are very worthy of investigation. The investigation draws the following conclusions. (1) Two failure mechanisms are distinguished, i. e. yielding of chord which occur usually with fixed type jacking unit, and buckling or yielding of braces which occures usually with floating type jacking unit. (2) In case of jack-up rigs investigated in this study, the max. wave force in survival condition is 1.6-1.8 times of design wave load and allowable penetration in punch-through condition is 4-5 m. This is believed to be typical for jack-up rigs. However, the present design may not be regarded as over conservative considering the many uncertainties involved. (3) In 3 legged jack-up rigs, failure of one leg would lead to total collapse of the rig. No redundancy is provided by the other two legs. Based on these conclusions, a simple analytical model is developed and simplified formulae are proposed to estimate the ultimate strength of jack-up rigs in the above mentioned two loading conditions depending upon the failure mechanism and the type of jacking unit.