We have investigated surface-enhanced plasmon resonance detection of DNA hybridization. Surface enhancement was based on the excitation of localized surface plasmon using subwavelength nanogratings, at a 300 nm period, coated with 24-mer ssDNA oligonucleotide, while optical signatures of DNA were amplified at the same time by gold nanoparticles conjugated with complementary ssDNA strands. When using nanoparticles of different sizes, maximum sensitivity enhancement, of more than 18 times, was obtained with nanoparticles of 20 nm diameter. This enhancement is mainly due to nanoparticle-associated signal amplification. Additional surface enhancement boosted the detection sensitivity by 57%. We have also confirmed the sensitivity enhancement to be linearly related to nanoparticle volume.