After the Fukushima No. 1 Nuclear Power Plant accident, high levels of radioactive cesium were detected in beef. Many prefectural governments decided to conduct blanket tests on meat from local beef cattle to prevent distribution of beef contaminated with radioactive cesium exceeding the provisional regulation value. In some cases, different concentrations of radioactive cesium were found in different cuts of beef from the same cows. These results raised questions about the reliability of the test results. Here, we investigated the reason for the differences in radioactive cesium concentration in different cuts of beef from the same cows. The concentrations of radioactive cesium in five different parts cuts of beef from three cows were negatively correlated with fat content, suggesting that the difference in radioactive cesium concentration is due to differences in fat content in the meat. In addition, our results showed that the concentration of radioactive cesium in muscle was more than 7-fold higher than that in fat in the same cow. These results suggested that it is necessary to use muscle for testing of radioactive cesium in cows.