Abstract: The experimentally - estimated modal parameters (frequencies, modes of natural vibration, corresponding damping values) realise the real immediate state of the observed bridge structure and its spatial behaviour. This is the reason for using the experimental modal parameters such as for an identification, an optimalization or a verification of a bridge design model. In addition, the apposite spatial model of the monitored bridge could be created directly from the experimental modal parameters of this bridge (so-called the modal model). There is possible to achieve with this model the apposite calculation of a dynamic response of an observed structure under a dynamic load. The response of the modal model under an influence of the live load can be calculated by the modal decomposition method. Today the stress is the main criterion of the load carrying capacity checking of a structure and its variability in time is the main criterion of the fatigue life checking. It is possible to evaluate the response of the modal model on a stress quantity scale. The calculated time histories of the stress in given details of a structure can be processed by one of classification methods (e.g. the rain flow method). Afterwards it is possible to make the estimation of influence of the observed dynamic load on a reduction of the fatigue life of the monitored structure based on the obtained results and the Palmgren - Miner hypothesis. The direct use of the modal model of the bridge for the calculation of its dynamic response is one of the interesting methods for studying of the vehicle/bridge interaction problem.