We performed an imaging analysis of 42K in Arabidopsis using real-time radioisotope imaging system(RRIS). First, we purified 42K from an 42Ar―42K generator. And then, we characterized RRIS performance by quantitatively determining 42K using standard series. As a result, the dynamic range for 42K was determined to be at least three orders of magnitude. Next, we evaluated the level of self-absorption in Arabidopsis organs by comparing the signal intensity detected using RRIS and the actual radioactivity detected by a gamma-counting method. There was no significant difference in detection efficiency between the thick bolt(stem) tissue and the thin leaf tissue. The reason for scarce self-absorption could be related to the relatively strong β ray emissions that have a maximum energy of 3525.4keV.