Total reactive oxygen species (ROS) signals in irradiated Arabidopsis plants were examined by electron paramagnetic resonance (EPR) analysis. At 10 kGy, the EPR signal intensity was highest in the root, whereas relatively low intensity levels were observed in the leaf and stem. The relative unit (r.u.) of control plants was 0.38 in the leaf, which was gradually increased to 0.51, 0.71, and 0.95 r.u. at 1, 5, and 10 kGy, respectively. In the stem, the intensity in all irradiated samples was lowest compared with that in other plant organs such as the leaf and root. The r.u. in the root sharply increased from 0.13 r.u. in control samples to 1.58 r.u. at 10 kGy, with 0.30-0.42 r.u. observed in 1-5 kGy irradiated samples. Stem and leaf extracts showed remarkably high levels of radical scavenging activity at 89.12 and 71.45%, respectively, compared with the very low level of activity in the root at 10.75%. These findings were in good agreement with the extraction yield of each plant organ, which was 20.0, 14.8, and 10.0% in the stem, leaf, and root, respectively. Order of EPR signal intensity and radical scavenging activity was as follows: EPR signal intensity: 1) leaf > root > stem at 1 and 5 kGy, 2) root > leaf > stem at 10 kGy; radical scavenging activity: stem > leaf > root. Results showed high or low levels of EPR signal intensity in different plant organs could be caused by the ROS removal power of extracts from different plant organs.