Biopharmaceutical products such as protein drugs and monoclonal antibodies (mAb) are currently of great interest with monoclonal antibody drugs being one of the fastest growing categories of biopharmaceutical products. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) has gained high interest for measuring mAb drugs in biological samples in recent years due to its high selectivity. Bevacizumab is a humanized immunoglobulin G (IgG) mAb drug against human vascular endothelial cell growth factor A (VEGF-A). It is used for treating many types of cancers. Recent studies have indicated that clinical outcomes vary among patients treated with bevacizumab and produce various side effects， such as vascular disorders. In this study， we developed an LC-MS/MS method to quantify bevacizumab concentration. We selected readily available and economic materials for sample preparation to facilitate its wider use in clinical fields.-Protein G was used to trap bevacizumab from human plasma. In place of an extended stable isotope-labeled internal standard (SIL-IS)， the IgG-based drug-IS tocilizumab was used because of its better calibration performance. The method was validated in terms of its precision， accuracy， linearity， and sensitivity. The accuracies which were expressed as percentage recoveries for three concentration levels were within 92.8 ± 3.2 to 112.7 ± 4.5%. Repeatability and intermediate precision in terms of peak area ratios were lower than 5.2 and 12.9% RSD， respectively. The application to patients' sample measurements revealed a wide individual variability of drug concentrations， and the proposed simple and general method may facilitate personalized medicine for improving therapeutic efficacy and safety. Graphical abstract ᅟ.