Isocyanate is a common name for chemical compounds containing one or more - NCO groups. There have been many studies showing that isocyanates pose a risk to human health when exposed and can cause occupational poisoning. When absorbed into the body, the isocyanates will be converted into the corresponding amines. Therefore, the determination of these diamines in urine will contribute to the assessment of exposure to isocyanates. This article presents a study on simultaneous determination of 4,4′-methylenedianiline (MDA), hexamethylene diamine (HDA), isophoron diamine (IPDA) in urine by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The method detection limit (MDL) were 0.074, 0.059, and 0.053 ng/mL for HDA, MDA, IPDA, respectively. The method quantitation limit (MQL) were 0.243, 0.194 and 0.177 ng/mL for HDA, MDA, IPDA, respectively. The method's recovery ranged from 86.38 to 105.3% with the repeatability RSDr < 6%. The method was successfully applied to simultaneously determine the content of HDA, MDA and IPDA in 30 urine samples of workers at auto repair garages in Bac Ninh province. The results showed that HDA, MDA and IPDA were detected in these samples with different concentrations.