Ni3Al/Ni multilayers have been grown by magnetron sputtering at room temperature. Structural characterisations were performed by means of X-ray diffraction and reflection. The indentation hardness of the samples as a function of the bilayer thickness has been studied. An increase in hardness up to 88% of the bulk hardness of Ni3Al is observed when bilayer thickness Lambda decreases to Lambda(max) approximate to 100 Angstrom. A further decrease in bilayer thickness results in a decrease in hardness. At Lambda > Lambda(max) the hardness variation is found to be compatible with the Hall-Petch relation. At Lambda < Lambda(max), the reduction in hardness is discussed in terms of structure and morphology of the layers. An increase of the mosaic spread of the grains as well as a decrease in density is observed at small Lambda.