4 ng/ml bFGF is indispensable for hESC cultured on mouse embryonic fibroblasts (MEF), withdrawal of bFGF drives the hESC to differentiate. In order to exploit effect of bFGF on MEF, we collected a series of MEF conditioned medium (bFGF-MCM) by co-culturing MEF with increasing bFGF concentrations: 0.03, 0.1, 0.3, 1 and 4 ng/ml. The primitivity of hESC cultured in bFGF-MCM was estimated by morphology and alkaline phosphatase staining. Compared with the control medium (medium conditioned without bFGF: MCM), percentage of undifferentiated colony was increased from 23% to 29%, 44%, 74%, 77% and 78%, respectively. However, percentage of undifferentiated colony in the blank medium (medium conditioned with bFGF but without MEF: bFGF-SR) was from 13% to 31%. This indicated that low concentration of bFGF acted on MEF and stimulated MEF producing effective conditioned medium for maintaining hESC. To identify active elements in the effective conditioned medium can help to understand mechanisms of hESC self-renewal.