In this research we reported the synthesis of oligonucleotide-silver nanoparticle (OSN) conjugates and demonstrated their use along with magnetic beads as biosensor for Escherichia coli detection under magnetic field condition. Oligonucleotide DNA probes were conjugated on silver nanoparticles using alkanethiols linker. Two kinds of alkanethiols linker, 11-mercaptoundodecanoic acid (11-MUDA) and 16-mercaptophexadecanoic acid (16-MHDA) were compared to get the best probe conjugation yield and OSN UV-Vis absorption spectra properties. Three different methods of Escherichia coli DNA isolation i.e. Chen and Kuo (1993), Phenol Chloroform Isoamylalcohol (PCI) extraction and boiling lysis were also compared to explore the performance of the biosensor towards the DNA target purity. Detection process through hybridization between the DNA probe and the target was carried out at 55 o C for 1 hour incubation time. The results showed that 16-MHDA gave higher conjugation yield and higher OSN UV-Vis absorption spectra than 11-MUDA. The biosensor was able to detect the presence of the DNA target which was isolated from the three isolation methods. The best detection signal was achieved by Chen and Kuo isolation method in which it could detect the presence of the DNA target up to 1.3 ng/μL.