A method for the accurate determination of aflatoxin B ₁ , aflatoxin B ₂ , aflatoxin G _{1 ,} and aflatoxin G ₂ in grains was established using isotope dilution-liquid chromatography-tandem mass spectrometry (ID-LC-MS/MS). ¹³ C- labeled aflatoxins (AFs) were used as internal standards. AFs in grains were extracted with a mixture of methanol/water (60/40; v/v) and purified using an immunoaffinity column (IAC) for adequate elimination of matrix interferences. The developed method exhibited excellent analytical performance over the studied range (0.5 to 4 µg/kg). The accuracy values ​​ranged from 97% to 103% for all AFs, with relative standard deviations for interday and intraday precision being below 3% and 4%, respectively. The expanded measurement uncertainty was less than 5% (with a 95% confidence level). The limits of detection (LOD: 0.002–0.013 µg/kg) and quantification (LOQ: 0.006–0.042 µg/kg) were substantially lower than the maximum level fixed by the European Union for grain cereals. Thus, the developed method can be used as a candidate reference method for the quantitation of AFs in grains. Moreover, the practical applicability of the developed method was further demonstrated by the determination of AFs in several grain samples.