Airborne radioactive particles released by the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in 2011 were collected with a cascade low-pressure impactor at the Japan Atomic Energy Agency (JAEA) in Tokai, Japan, 114 km south of the FDNPP. Size-fractionated samples were collected twice, in the periods of March 17 April 1, 2011, and May 9-13, 2011. These size-fractionated samplings were carried out in the earliest days at a short distance from the FDNPP. Radioactivity of short-lived nuclides (several ten days of half-life) was determined as well as Cs-134 and Cs-137. The elemental composition of size-fractionated samples was also measured. In the first collection, the activity median aerodynamic diameter (AMAD) of Te-129m, Ba-140, (CS)-C-134, Cs-136 and Cs-137 was 1.5-1.6 mu m, while the diameter of I-131 was 0.45 mu m. The diameters of Cs-134 and Cs-137 in the second collection were expressed as three peaks at <0.5 mu m, 0.94 mu m, and 7.8 mu m. The Cs-134/Cs-137 ratio of the first collection was 1.02 in total, but the ratio in the fine fractions was 0.91. A distribution map of Cs-134/Cs-137 - Cs-136/Cs-137 ratios was helpful in understanding the change of radioactive Cs composition. The Cs composition of size fractions <0.43 mu m and the composition in the 1.1-2.1 mu m range (including the AMAD of 1.5-1.6 mu m) were similar to the calculated compositions of fuels in the reactors No. 1 and No. 3 at the FDNPP using the ORIGEN-II code. The Cs composition collected in May, 2011 was similar to the calculation results of reactor No. 2 fuel composition. The change of Cs composition implies that the radioactive Cs was released from the three reactors at the FDNPP via different processes. (C) 2014 Elsevier Ltd. All rights reserved.