This research is about surface-enhanced Raman spectroscopy based on the gap-plasmonic effects between the silver nanoisland (AgNI) substrate and gold nanoparticles (AuNPs). With calculation, we prove that plasmonic-coupling phenomena between AuNPs and AgNIs were formed, which eventually affect to the signal enhancements, and we simulate the field enhancement according to the AuNPs position on the AgNI substrates. Consequently, we experimentally confirm the Raman signal enhancement using target as AuNP attached DNA, which were distributed on the AgNIs substrate randomly. Raman spectra measured on the AgNI substrate exhibit approximately 20-fold signal enhancements compare to the signals measured on a uniform silver film, and the experimental spectra agreed well with the results of simulation. This method has merit in that significant Raman signal enhancements can be achieved for large areas without a complicated nano-lithographic process.