KamLAND-PICO project aims to search for WIMPs dark matter by means of NaI(Tl) scintillator. To investigate the WIMPs candidate whose cross section is as small as $10^{-9}$ pb, a pure NaI(Tl) crystal was developed by chemical processing and taking car
e of surroundings. The concentration of U and Th chain was reduced to $5.4pm0.9$ ppt and $3.3pm2.2$ ppt, respectively. It should be remarked that the concentration of $^{210}$Pb which was difficult to reduce reached to the high purity as $58pm26$ $mu$Bq/kg.
