The aim of this study was to explore a safe method collecting peripheral blood stem/progenitor cell (PBSPC) from the infants of body weight less than 20 kg by using the COBE Spectra Blood Cell Separator through Auto-PBSC procedure. After washing tube by normal saline, one unit of irradiated RBC was infused into the apheresis set. When the collection terminated, only the concentrated RBC in the apheresis set was returned to the infant. The peripheral mononuclear cells (PBMNCs) and CD34+ cells were counted, the cell viability was determined. The results showed that 13 PBSPC collections were carried out successfully from 7 infants of body weight<20 kg. The average count of MNCs was 4.44x10(8)/kg [(3.46-6.45)x10(8)/kg], the CD34+ count was 2.20x10(6)/kg [(1.34-3.79)x10(6)/kg] and the cell viability was 98.45% (97%-100%) respectively. The vital signs of all the infants went smoothly during collection of PBSPCs. In conclusion, with the aid of COBE Spectra blood cell separator and other measures, the collection of PBSPCs from infants of body weight<20 kg is safe and effective, the PBMNCs containing enough PBSPC can be harvested for transplantation.