In this study, stable carbon and oxygen isotope data were used to determine the country of origin of imported beef circulating in the Korean market. Using carbon isotopes closely related to the animal diets, Korean beef could easily be distinguished from United States (US) and New Zealand beef. However, oxygen isotopes were more effective in distinguishing the Korean from Australian beef. The oxygen isotopes have specific characteristics reflecting the latitude where cattle feed. Statistical analyses indicate that analyzing both carbon and oxygen isotopes is very useful to assess the country of origin of beef sold in the Korean market. Additionally, we compared the carbon isotopic compositions of defatted and non-defatted beef to evaluate the effect of fat content on carbon isotope values. Our results suggest that by using muscles or tissues that have small amounts of fat, carbon isotope analysis of beef samples may be performed without the inconvenient conventional pretreatment procedure, allowing for easier and faster isotopic analysis.