This study aimed to investigate the anti-allergic activity and the mechanism of action of Crassirhizomae rhizoma (CR). The extract of CR exhibited potent inhibitory activity in mast cells; its $IC_{50}$ values were $31.2{pm}1.5{mu}g/m{ell}$ for rat basophile leukemia (RBL)-2H3 mast cells and $51.5{pm}2.1{mu}g/m{ell}$ for bone marrow-derived mast cells by antigen stimulation. It also suppressed the expression of TNF-${alpha}$ and IL-4 mRNAs in RBL-2H3 cells. In an in-vivo animal allergy model, it inhibited a local allergic reaction, passive cutaneous anaphylaxis (PCA), in a dose-dependent manner. With regard to the mechanism of action, CR inhibited the activating phosphorylation of Syk kinase, a key signaling protein for the activation of mast cells. Taken together, these results strongly suggested that the anti-allergic activity of CR is mediated through the inhibition of histamine release and allergic cytokine production by the inhibition of Syk in mast cells.