Disruption of the blood-brain-barrier (BBB), a crucial event for the entry of inflammatory cells into brain, is a prerequisite for the pathogenesis of many cerebral diseases. Oxidative stress is one of the well-known factors that accounts for the leakage of the BBB, which can be alleviated by the activation of the nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathway. Isoastragaloside I (ISOI) is a cycloartane glycoside isolated from Radix Astragali, a clinical widely used traditional Chinese medicine. In the present study, the effect of ISOI on LPS-induced bEnd.3 cells was assessed and the underlying molecular mechanisms were investigated. Our results showed that ISOI pre-treatment inhibited the reduction of TEER, prevented the increase of NaF extravasation and ROS production, and rescued the down-regulated tight-junction (TJ) protein level in bEnd.3 cells stimulated with LPS. Meanwhile, it significantly activated Nrf2 activity, increased its nuclear translocation, thereby, enhanced the expression of its downstream molecules such as NQO1 and HO-1. When Nrf2 was silenced by siRNA, ISOI could not rescue the decrease of TJ proteins induced by LPS any more. In addition, ISOI pre-treatment reduced the elevation of VCAM-1, IL-1β and TNF-α at either protein or mRNA levels, and abrogated the increased adhesion of the monocyte JAWS II cells onto bEnd.3 cells stimulated with LPS. Taken together, these results indicated that ISOI could protect the integrity of the BBB under inflammatory conditions, which was probably mediated through activating the Nrf2 antioxidant pathway.