A high order method for measuring urea concentrations in milk and milk powder was developed. The method can be applied to certify the concentration of urea in some new milk and milk powder CRMs. This high accurate method for analysis of milk is valuable given the inherent challenges associated with the complexity of the sample matrix. A measurement procedure based on gas chromatography/isotope dilution mass spectrometry (GC/IDMS) was developed. Samples were pre-treated with acetonitrile to remove proteins and the method was applied to determine urea concentrations in milk and milk powder. Excellent precision was obtained, with within- and between-set coefficients of variation of 0.15–0.46 and 0.18–0.65%, respectively. The measurement uncertainty is evaluated. The method can trace to mass.