Seaweeds can efficiently accumulate almost all the elements in seawater, and serve as good experimental material for elemental analysis. Undaria pinnatifida Suringar, a favorite edible alga, can form distinctive reproductive organs, i.e. sporophylls (Chihara, 1975; Chapman and Chapman, 1973). Sporophyll of Undaria pinnatifida is believed to have great potential for enhancing human health and regarded as potent folklore medicine (Nisizawa, 1989). Some experimental data proved that sporophyll of Undaria pinnatifida is beneficial as cancer-preventive and cardiovascular protective agents (Furusawa and Furusawa, 1985; Oishi, 1993 ). Elements accumulated in the sporophyll can explain the physiological bioactivities of the sporophyll. So it is interesting to know the special elements and their localization in the sporophyll. X-ray analytical microscopy, a rapid and visual method for analysis of elemental composition, can provide an X-ray transmission image of a sample and fluorescent X-ray analysis of elemental mapping at the same time (Hosokawa, 1995). Using such a non-destructive technique, the distribution of certain elements can be observed from the formed imaged directly if the element is high enough to show a clear image. Here, the authors report the results of their study on the location of major elements such as potassium and calcium in the sporophyll of Undaria pinnatifida.