Fluorescence spectra and absorption spectra of carvacrol, an active component of Chinese herbal medicines, have been studied. The ionization constant and fluorescence quantum yield of carvacrol were measured according to spectral data. Under the condition of pH<2.0, fluorescence intensity of carvacrol increases with the increase in pH value. In the range of pH 2.0~8.0, carvacrol gives a strong and steady fluorescence with maximum excitation wavelength 278 nm and emission wavelength 306 nm. When pH>8.0, the fluorescence intensity decreases with the increase in pH value. Ionization constant of carvacrol was measured to be pKa=10.44±0.06 using a pH-absorbance method; and pKa=10.40±0.04 using a pH-fluorescence method. Fluorescence intensity of carvacrol was remarkably enhanced when methanol was added into its aqueous solution. Using L-tryptophane as a reference, the fluorescence quantum yield of carvacrol aqueous solution was measured to be 0.121 at excitation wavelength 278 nm; while in a solution containing 80% methanol, the quantum yield was measured to be 0.324.