The rhizome of Cnidium officinale (Umbelliferae) (known as Senkyu in Japan; COR) has been used as a crude drug in Japanese Kampo formulas, such as Jumihaidokuto (to treat eczema and urticaria) and Kakkontokasenkyushin’i (to treat rhinitis). COR contains phthalides, which are thought to be potent principal constituents. Few studies have been reported about the comparison of anti-inflammatory activity of COR constituents. We aimed to identify the constituents in COR and compare their anti-inflammatory activity. COR was extracted with methanol and fractionated into ethyl acetate (EtOAc)-soluble, n-butanol-soluble, and water-soluble fractions. Primary cultured rat hepatocytes were used to assess anti-inflammatory activity by monitoring the interleukin (IL)-1β-induced production of nitric oxide (NO), an inflammatory mediator. The EtOAc-soluble fraction significantly suppressed NO production without showing cytotoxicity in IL-1β-treated hepatocytes, whereas the n-butanol-soluble fraction showed less potency, and the water-soluble fraction did not significantly affect the NO levels. Four constituents were isolated from the EtOAc-soluble fraction and identified as senkyunolide A, (3S)-butylphthalide, neocnidilide, and cnidilide. Among these phthalides and (Z)-ligustilide, senkyunolide A and (Z)-ligustilide efficiently suppressed NO production in hepatocytes, whereas the others showed less potency in the suppression of NO production. Furthermore, senkyunolide A decreased the levels of the inducible nitric oxide synthase (iNOS) protein and mRNA, as well as the levels of mRNAs encoding proinflammatory cytokines (e.g., tumor necrosis factor α) and chemokine C–C motif ligand 20. These results suggest that senkyunolide A may cause the anti-inflammatory and hepatoprotective effects of COR by suppressing the genes involved in inflammation.