Okitsu Wase, one of the Citrus unshiu Marc., commonly produced in Japan,. contains flavonoids, such as hesperidin, nobiletin and tangeretin in the peel. The flavonoids, secondary metabolites in several plant species, are recently gaining attention for their beneficial properties for human health, and some research is conducted to identify the biosynthetic path or affecting factors for the accumulation of those compounds. However, biosynthetic paths and influencing factors are not fully identified. Since secondary metabolites have essential roles in tolerant of environmental stresses, this research aimed to determine the effect of solar radiation on the flavonoids in the peel. Thus, accumulated solar radiation, fruit position (height level and direction) and the quantities of hesperidin, nobiletin and tangeretin contained in the peels of Okitsu Wase were measured in the fruit samples taken from five different trees in the same field. In each tree, the samples were taken from three different height levels (top, middle, bottom) and four directions (north, south, west, east). After the peels of fruits were separated, and freeze- dried,. the target compounds were extracted from dried peels by methanol, and then quantitative analysis was conducted using HPLC. The results indicated similar quantities of hesperidin, nobiletin and tangeretin in the peels of Okitsu Wase regardless of the fruit position (height level and direction),. although the quantity of accumulated solar radiation to the fruit position was different at each height level. Moreover, the quantities of hesperidin, nobiletin and tangeretin contained in the peels of Okitsu Wase were constant among the five different trees. Thus, the result indicated that the effect of solar radiation on the fruits of Okitsu Wase at the growing stage is less likely to affect the quantity of flavonoids such as hesperidin, nobiletin and tangeretin contained in the peels.