Low-molecular-weight thiols play important roles in a variety of pathological processes and are closely associated with a wide range of diseases. In this study, a selective and sensitive method was developed for the simultaneous determination of all the 7 thiols occurring in the transsulfuration pathway (Cysteine (Cys), homocysteine (Hcys), glutathione (GSH), N-acetylcysteine (Nac), cysteinylglycine (CysGly), glutamylcysteine (GluCys) and cysteamine (CA)) in human serum by in-vitro stable isotope labeling - dispersive solid phase extraction - liquid chromatography - tandem mass spectrometry analysis (IL-DSPE-LC-MS/MS). In the proposed method, a pair of stable isotope-labeling reagents, BQB (to-bromoacetonylquinolinium bromide) and BQB-D-7, were utilized to label thiols in human serum samples and thiol standards, respectively. The BQB labeled thiols which carry a positive charge were extracted and purified with C-8-SO3H-based DSPE followed by LC-MS/MS analysis. Good linearities for 7 thiols occurring in the transsulfuration pathway were obtained with the coefficient of determination (R-2) > 0.9901. The limits of detection (LODs) were in the range of 0.7-6.0 nmol/L. The method was further applied to investigate the contents change of 7 thiols in human serum samples of type 2 diabetes mellitus (T2DM) patients and breast cancer (BC) patients. The results showed that the contents of these thiols occurring in the transsulfuration pathway significantly changed and were highly diseases-related. In addition, partial least squares discriminant analysis (PLS-DA) suggested excellent classification performance between patients and healthy controls. The findings indicated that these significantly changed thiols occurring in the transsulfuration pathway in T2DM patients and BC patients might serve as the indicator for the diagnosis of T2DM and BC. Taken together, the developed IL-DSPE-LC-MS/MS method provides a promising tool for the sensitive analysis of thiols from complex biological samples, which may promote the in-depth investigation of the functions of thiols.