  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 care of surroundings. The concentration of U and Th chain
was reduced to $5.4\pm0.9$ ppt and $3.3\pm2.2$ ppt, respectively. It should be
remarked that the concentration of $^{210}$Pb which was difficult to reduce
reached to the high purity as $58\pm26$ $\mu$Bq/kg.
