The simple Hg(II)-detection system was studied and developed using dithizone nanofiber loaded membrane. Dithizone is one of the most frequently used chemical substances for Hg(II) ions extraction in water. Hg(II)-dithizone complex susbtance [Hg(HDith)2] is stable in acidic condition and presenting with the orange-brown color. To prepare dithizone nanoloaded membrane, dithizone solution (in acetone) was mixed in aqueous solution with vigorous stirring. After dithizone nanofiber dispersion was formed, the agent during nanofiber preparation step and keeping dithizone membrane in aluminum bag as a reducing absorber under vacuum would improve the stability of dithizone against oxidation from air and/or light. The Hg(II)-detection was performed by the colorimetric equipment. It was found that the concentration range of Hg(II)-detection can be altered with sample volume. By this technique, the detection limit can be achieved as low as 0.057 ppb with sufficient precision and repeatability. Practically, it could be applied to Hg(II)-determination in real samples from environment such as artificial waste water, river water and sea water.