In this article, we report the use of DNA molecular ruler to modulate the PL emission of GaAs with SP enhancement. A well-defined GNPs/GaAs hybrid surface with DNA junction has been designed and demonstrated, as schematically represented in Fig. 1. This experimental approach was based on sequential steps of DNA functionalization, DNA hybridization and GNPs self-assembly on GaAs, which comprises two steps of thiol-chemistry reaction (Au-S, Ga-S or As-S) appended a relatively rigid dsDNA onto GaAs and GNPs, respectively [23]. This simple method offers a facile nonlithograhic strategy to modify large flat GaAs substrates with GNPs. Next, we found that the DNA-mediated self-assembly of GNPs onto GaAs could enhance the PL emission from GaAs. It is proposed that the DNA-length dependent PL enhancement is resulted from the known as the mechanism of metal-enhanced fluorescence.