The peel of jinkyool (Citrus sunki Hort. ex Tanaka) has been widely used in traditional Asian medicine for treatment of a number of diseases, including indigestion and bronchial asthma. In the present study, we compared the flavonoids content and anti-inflammatory activities of ethanolic peel extracts from both immature and mature fruits. Comparing to the mature peel extract (MPE), the immature peel extract (IPE) contained more abundant flavonoids. IPE more effectively suppressed intracellular reactive oxygen species (ROS) generation (IPE, $67.6{pm}1.2$%; MPE, $78.9{pm}2.4$% at 300 ${mu}g/mL$ of control), nitric oxide (NO) production, and inducible NO synthase (iNOS) and cyclooxygenase-2 (COX-2) protein expression, as well as interleukin (IL)-$1{eta}$, IL-6, and tumor necrosis factor-${alpha}$ (TNF-${alpha}$) mRNA expression in lipopolysaccharide-stimulated RAW 264.7 cells. Additionally, IPE reduced c-Jun $NH_2$-terminal kinase (JNK) and p38 mitogen-activated kinase (p38) activation, suggesting that IPE may exert an anti-inflammatory effect by suppressing pro-inflammatory cytokines that suppress activation of mitogen-activated protein kinases (MAPKs). Taken together, these results indicate that IPE has potential for use as an anti-inflammatory agent.