Abstract: In this paper a crucible-thermionic ionizer (CTI) for precisely determin-ing isotopic ratio of lithium is described. Using CTI to analyse 0.1 micro-gram of lithium (LiCI) an external precision better than ±0.2% (RSD) is ob-tained duriug the filament temperature is about 1600°C and the variation of operating temperature is within ±100°C. Three laboratory standards are pre-pared by mixing two solutions which contain high abundances of ~6Li or ~7Li.The instrument has been calibrated with those three standards. The standarddeviation of calibration factor is 0.08%.