As citrus peels are increasingly used as valuable by-products, it is necessary to provide a more comprehensive view on their phytochemicals and discuss how to improve their use. Phenolic acids, flavonoids, furanocoumarins, total phenolic and flavonoid contents in seven citrus peels, bitter orange, grapefruit, lemon, lime, orange, satsuma mandarin, and yuzu were evaluated. The satsuma mandarin and yuzu peels included various phenolic acids and flavonoid glycosides with even distributions, and they were free of furanocoumarins. The bitter orange and grapefruit peels had a few types of flavonoid glycosides and furanocoumarins. The lemon, lime, and orange peels were relatively low in phenolic acids and flavonoid glycosides, and furanocoumarins were found in lemon and lime peels. A high-temperature steaming process at 120 °C was applied to enhance the benefits of the citrus peels. More phenolic acids were extracted at the beginning of processing, and flavonoid glycosides were partially degraded. As the process continued, the diminished glycoside content was restored owing to an increase in extraction efficiency. The flavonoid aglycones were significantly produced and peaked at 8 h, and the furanocoumarins were reduced after 4 h. The antioxidant capacity by DPPH and FRAP assays increased by 2.33 times at 8 h on average.