The method of "in situ tensile testing in SEM" is suitable for investigations of fracture mechanisms because it enables to observe and document deformation processes directly, thank to which the initiation and development of plastic deformation and fracture can be reliably described. The deformation and fracture mechanisms of Cu-Al2O3 nanomaterials with 5 vol.% of Al2O3 phase has been analyzed using technique of the "in situ tensile testing in SEM." It has been shown that the deformation process causes break-up of large Al2O3 particles and decohesion of smaller ones. The final fracture path is influenced also by boundaries of nanograins, through which the principal crack propagates towards the sample exterior surface. Based on the experimental observations, a model of damage and/or fracture mechanisms has been proposed.