Mesenchymal stem cells (MSCs) from various animals undergo spontaneous transformation in vitro,establishing some malignant characteristics. However,this phenomenon seems seldom appearing in human (h)MSCs. To address the question whether the hMSCs really do not undergo the spontaneous transformation and why,the present study compared MSCs from two species under the same conditions, the commercialized primary hMSCs whose in vitro life span is very uniform, and the rat (r)MSCs whose spontaneous transformation in vitro is well defined.It was demonstrated that in rMSCs, there were small numbers of re-proliferating cells appearing after a substantial senescent period. These “senescence-escaped”rMSCs were highly proliferative and did not show any sign of growth arrest during the following subcultures upto observed passage 32. Whereas after entering senescence, hMSCs no longer re-proliferated and finally died from apoptosis. Compared with rMSCs, the hMSCs possessed a much shorter telomere, and lacked both telomerase reverse transcriptase expression and telomerase activity. When proliferating from pre-senescent to senescent stages,the hMSCs had a greater loss of relative telomere length(51 % in hMSC vs. 15 % in rMSC), but both cells displayed a similar average telomere shortening per population doubling (0.50 ± 0.06 kb in rMSC vs. 0.49 ± 0.06 kbin hMSC; p > 0.05), indicating that the greater relative shortening of the hMSC telomeres was due to their original shorter length, rather than lack of telomere maintenance mechanisms. In conclusion, the hMSCs do not spontaneously initiate transformation, because they cannot escape senescence. This is particularly due to their much shorter telomere.