In order to develop simple sensing system for pork adulteration in processed meats, the DNA biosensor based on 20 nm gold nanoparticles (AuNPs) with aggregation property has been developed as a simple colorimetric detection of target DNA. Adsorption of single-stranded (ss) DNA on AuNPs protects the particles against salt-induced aggregation. However, mixing and annealing of a 27-nucleotide (nt) ssDNA probe on AuNPs with denatured DNA of different processed meats differentiated between perfectly matched and mismatch hybridization at a critical annealing temperature (55 °C). The AuNPs change color from red to purple, in 10 mM phosphate buffer saline (PBS). At a hybridizing temperature (55 °C), non-target mismatched DNA provided hybridization products allowing probe to be free and adsorbed to AuNPs. This prevented AuNPs from salt-induced aggregation as the color still red. In matched DNA, hybridization would occurred, allowing probe to be occupied. This facilitated AuNPs from salt-induced aggregation and induced colorimetric change of particles from red to purple. These signals could be observed easily with naked eye. This label-free DNA nanobiosensor should find applications in food analysis and other DNA based screening.