Mechanical properties of filled polymers are dependent on a lot of parameters : matrix properties, particles characteristics (nature, size, shape, size distribution), constituent volume fraction and particle-particle and matrix-filler interactions. In this work, mainly devoted to polymer-filler interactions, the tensile behaviour of calcium carbonate-filled polystyrene is examined for different kinds of filler surface modification : carboxylic acid adsorption and polystyrene or polybutyl acrylate grafting. Experimental relative tensile strength of the composite varies mainly with the matrix proportion in the fracture surface and with the matrix-filler interactions : adhesion level and matrix-particle stress transfer. The model enables the calculation of dewetting angles which are a very good representation of the filler-matrix adhesion level.