The PICO-LON project aims at search for cold dark matter by means of highly radio-pure and large volume NaI(Tl) scintillator. The Nal powder was purified by chemical processing to remove lead isotopes and selecting a high purity graphite crucible. The concentrations of radioactive impurities of Ra-226 and Th-228 were effectively reduced to 58 +/- 4 mu Bq/kg and 1.5 +/- 1.9 beta Bq/kg, respectively. It should be remarked that the concentration of zio-, r D which is crucial for the sensitivity to dark matter, was reduced to 24 +/- 2 beta Bq/kg. The total background rate at 10 keV(ee) was as low as 8 keV(-1)kg(-1)day(-1), which was sufficiently low to search for dark matter. Further purification of NaI(Tl) ingot and future prospect of PICO-LON project is discussed.