In the human body, vitamin K 1 is important for bone and cardiovascular health and blood coagulation. To assess the correlation between vitamin K 1 intake and health outcomes, the accurate determination of the amounts of vitamin K 1 in green leafy vegetables, which are its major source in the diet, is needed. In this study, an accurate method for quantifying naturally occurring trans - and cis -vitamin K 1 in spinach and kimchi cabbage was developed on the basis of isotope dilution-liquid chromatography/tandem mass spectrometry (ID-LC/MS/MS). A C30 column was employed for proper separation of trans - and cis -vitamin K 1 isomers, and vitamin K 1 - d 7 was used as an internal standard. The developed method was validated by measuring gravimetrically fortified samples, and its performance parameters were evaluated. The measured results agreed with the gravimetric results with a difference of less than 3%. The repeatability and reproducibility of the vitamin K 1 analysis were less than 2% relative standard deviation, indicating that the method had a higher-order metrological quality as a reference method.