Al/Si3N4 multilayers with different individual layer thicknesses (10 nm–500 nm) fabricated by magnetron sputtering on Si substrate were tested by using nanoindentation. The mechanical properties and deformation behavior change clearly with the individual layer thickness t. Hardness and mechanical strength increase in a Hall–Petch form with decreasing individual layer thickness for t>=100 nm, and also decrease in a deviated Hall–Petch form with further decreasing individual layer thickness for t<=100 nm. It is found that the Hall–Petch constant of metal/ceramics system is much higher than those of metal/ metal systems. The deformation mechanism in the Al layers changes from dislocation pileups dominated to Orowan-type bowing of individual dislocations and with concurrence of grain boundary sliding when the layer thickness is less than 100 nm. Delamination occurs as well at the interface between the Al and Si3N4 of the multilayers with layer thicknesses of 50 nm and 100 nm, because the local stress concentration is excess to the critical stress for the delamination during the indentation. Ceramic layers (Si3N4) can bear severe deformation without breaking under compression but not under tension.