The accident at the Fukushima Daiichi Nuclear Power Station (F1NPS) in March 2011 emitted 1.2 × 1016 Bq of cesium-137 (137Cs) into the surrounding environment. Radioactive substances, including 137Cs, were deposited onto forested areas in the northeastern region of Japan. To clarify the mechanisms of dispersion and export of 137Cs, within and from a forest ecosystem, we have conducted intensive field observations on hydrological processes and the 137Cs movement and storage in a forested headwater catchment in an area ~50 km from F1NPS. Two major pathways of 137Cs transport were focused: 1) through the hydrological processes with dissolved and particulate or colloidal forms, and 2) by dispersion through the food web in the forest-stream ecological continuum. The 137Cs concentrations of stream waters were monitored. Various aquatic and terrestrial organisms were periodically sampled to measure their 137Cs concentrations. The results indicate that the major form of exported 137Cs is via suspended solid in the streamflow. Thus, high flows generated by a storm event accelerated strongly the transportation of 137Cs from the forested catchments. Estimation of 137Cs export from the forested catchments requires precise evaluation of the high water flow during storm events. On the other hand, dissolved form, especially mineral ion form of 137Cs were dominant in through fall and stemflow. Because the biggest pool of 137Cs in the forested ecosystem was the accumulated litters and detritus on the forest floor, 137Cs dispersion through food web was quicker through the detritus food chain than through the grazing food chain. 137Cs concentration of leaves (1.1k Bq/kg), barks (4.4-22.5 kBq/kg) woods (0.1-0.6 kBq/kg) and litters (1.3 kBq/kg) of a dominant deciduous tree (Quercus serrata) suggested that internal cycling of 137Cs have already occurred between surface soils including litter layer and plants.