The main objective of the current study was to develop a universal method forprotein binding assay of complicated herbal components, and investigate the possible relationship between compound polarity and protein binding by taking Schisadra lignans as an example.Firstly, the rat, dog and human plasma were spiked with three different concentrations of Schisandra chinensis extract (SLE),and ultramicrofiltration was used to obtain the unbound ingredients.Secondly, totally 31 sehisandra lignans in total plasma and ultrafilter.fluid were measured by LC-IT-TOF/MS.Lastly, a relative exposure approach, which entailed calculating the relative concentrations of each schisandra lignan from the corresponding calibration equation created from the calibration samples spiked with the stock solution of SLE, was applied in order to overcome the drawback of the lacking of authentic standards.Our results showed that Schisandra lignans exhibited a high capability to bind with plasma protein and no species differences were observed.Furthermore,protein binding ratio of the lignans components increased proportionally with their individual chromatographic retention time, which indicated that the ratio of protein binding of lignans might increase accordingly with the decreasing polarity.Our study suggested that the compound polarity might be an important factor affecting the plasma protein binding of herbal components.