Cesium was recovered from plant samples obtained from Fukushima prefecture. The isotopic ratios of Cs-134/Cs-137 and Cs-135/Cs-137 were analyzed by thermal ionization mass spectrometry with a single filament method using a TaO activator. Samples containing 5 Bq of Cs-137 were analyzed with typical analytical errors of approximately 0.5% for Cs-134/Cs-137 and approximately 0.1% for Cs-135/Cs-137. Measurements of both ratios showed profiles that were characteristic of the measurements of among other environmental samples reported in the literature. The results showed the isotopic ratios of Cs-134/Cs-137 and Cs-135/Cs-137 were applicable for the source analysis of radioactive Cs in Fukushima prefecture.