Substrates with extremely high surface-enhanced Raman scattering (SERS) activity are fabricated based on arrays of Ag-nanoparticles on the surface of anodic aluminum oxide (AAO) film. The gaps between the Ag-nanoparticles are precisely varied in the unprecedented sub-10-nm scale and the SERS enhancing power increases dramatically with the narrowing of the gaps. The results provide quantitative evidence of collective SERS effect and confirm predictions of interparticle-coupling induced Raman enhancement. The Ag/AAO substrate’s uniform and highly reproducible SERS-active properties as well as its wide dynamical range facilitate the use of SERS for many chemical/biological sensing applications.