Surface-enhanced Raman scattering (SERS) can provide “fingerprint” characteristic of molecular vibration with high sensitivity and rapid response, and thus is a promising analytical technique. The intensity of molecular Raman signal will be amplified by several orders and the so-called SERS effect occurs when molecules locate on the surface of noble metal or some semiconductor nanostructures. Therefore, nanostructured SERS substrates are essential to SERS effect. Here we report that Au micro/nanoparticle arrays are fabricated on a Si wafer by a simple electrodeposition approach. There are many nanoscale gaps between the neighboring nanoparticles, which can provide SERS "hot spots" upon laser excitation, therefore the arrays of Au micro/nanoparticles have high SERS sensitivity. Additionally, the Au micro/nanostructures are highly stable in both chemical and structure. The as-prepared Au micro/nanostructures showed high SERS sensitivity to ultratrace (10-12 M) rhodamine 6G (R6G) with good SERS signal uniformity. Successful detection of toxic methyl parathion with a low concentration in water was achieved by using the as-prepared Au micro/nanostructure arrays as SERS substrates. This result shows the promising application of our Au micro/nanostructure arrays in SERS monitoring of toxic organic environmental pollutants.