his paper has demonstrated, for the first time, the synthesis of yellowish-brown colored stable As(0) sol by borohydride-reduction of aqueous sodium arsenite maintained at pH 7-9 under ambient condition. Scanning electron microscopic (SEM) images reveal the formation of spherical As(0) nanoparticles having size in the range 67?2 nm. The transmission electron microscopic (TEM) studies show the particle size lie within the range 60?3 nm. The X-ray photoelectron spectroscopy (XPS) indicates the zero oxidation state of arsenic. The dynamic light scattering (DLS) studies show that the average particle size is around 45 nm. The absorbance of yellowish-brown As(0) sol at 300 nm shows excellent correlation with arsenic concentration in the range 15.6 – 45.4 ? 10-5 M. The method is simple and reproducible. Also by measuring the turbidity, the quantification of arsenic is possible in the range mentioned. The effects of various ions and temperature on the formation of arsenic nanosol have been reported. The as-prepared As(0) nanoparticles may have potential applications in medicine and sensors.