Model updating techniques are used to update a "nite element model of a structure so that an updated model predicts the dynamics of a structure more accurately. The application of such an updated model in dynamic design demands that it also predict the e!ects of structural modi"cations with a reasonable accuracy. This paper deals with updating of a "nite element model of a structure and its subsequent use for predicting the e!ects of structural modi"cations. Updated models have been obtained by a direct model updating method and by an iterative method of model updating based on the frequency response function (FRF) data. The suitability of updated models for predicting the e!ect of structural modi"cations is evaluated by some computer and laboratory experiments. First a study is performed using a simulated "xed}"xed beam. Cases of complete, incomplete and noisy data are considered. Updated models are obtained by the direct and the FRF-based method in each of these cases. These models are then used for predicting the changes in the dynamic characteristics brought about due to a mass and a beam modi"cation. The simulated study is followed by a study involving actual measured data for the case of an F-shape test structure. The updated "nite element models for this structure are obtained again by the direct and the FRF-based method. Structural modi"cations in terms of mass and beam modi"cations are then introduced to evaluate the updated model for its usefulness in dynamic design. It is found that the predictions based on the iterative method based updated model are reasonably accurate and, therefore, this updated model can be used with reasonable accuracy to perform dynamic design. The predictions on the basis of the direct method based updated model are found to be reasonably accurate in the lower portion of the updating frequency range but the predictions are in a signi"cant error in the remaining portion of the updating frequency range. It is concluded that the updated models that are closer to the structure physically are likely to perform better in predicting the e!ects of structural modi"cation.