On the basic of the synthesis of CdTe/CdS core/shell quantum dots (QDs) in aqueous phase, we study ultrafast carrier dynamic behaviors of CdTe/CdS QDs with different shell thicknesses, by using femtosecond time-resolved transmission spectroscopy. The results show that both the buildup time of the lowest exciton state and its following relaxation time become longer as CdS shell thickness increases. The results also indicate that the buildup rate of a space-separate exciton is slower than that of a space-overlapped exciton, and that carrier relaxation changes from surface-defect related mechanism into surface-passivated charge separation regime, which is consistent with steady-state absorption and photoluminescence experiments.