The selection of dominant human umbilical cord blood-derived mesenchymal stem cells (CHUCBMSCs) is a prerequisite for cell survival and also a key factor for avoiding senescence in MSCs as potential therapeutic agents in regenerative medicine. To facilitate this selection, we devised a novel Stem Cell Scoring System (SCSS) based on multi-lineage differentiation, growth rate and morphology analysis, cell cycle passaging, immunophenotypic analysis in combination with the evaluation of senescence-associated gene expression, reactive oxygen species formation and the effects of siRNA against senescence-associated genes including SA-β-Gal. The high scoring dominant cells (Group A; score 7~10) survived 16 passages, compared to 12 passages for the low scoring group (Group C; score 1~4). The percentage of lineages able to differentiate in vitro into osteogenic, adipogenic and neurogenic lineages were approximately 93.5%, 67.5% and 4.2% for high scoring cells, respectively. The differentiation capabilities of all lineages decreased with increasing passages, and the differentiation of the adipogenic and osteogenic lineages decreased more than that of the neurogenic lineage. A quantification of cell cycle inhibitor expression in MSCs showed that senescence is accompanied by increased expression of p21 and MMP1 (as indicated by lower SCSS), but not TRF1. Low scoring cells showed increased SA-β-gal staining and produced more ROS compared to high scoring cells. The scores of siRNA transfected cells increased, as expected; expression was more inhibited in high scoring cells than in low scoring cells. These findings suggest that hUCB-MSCs with high SCSS scores may be useful in regeneration processes or in transplantation therapies.