Soil Organic Matter (SOM) is one of major C pools in biosphere, which play a key role in geochemical C cycle related directly to global warming and in maintenance of soil fertility. To understand the dynamics of SOM, the relationship between the stability and chemical structure is essential information. Although it is generally thought that humic substances are non-labile, humic substances have various chemical structures, consequently their degradation rates are also various. Initial humic substances may become refractory by losing their aliphatic moieties while polymerization-condensation and oxidation progress during a long period. Humic acids are a major fraction of humic substances that account for several to 40% of total SOM. The stability of humic acids has been considered to increase with increasing degree of humification, which is frequently evaluated from variables expressing the degree of darkening, such as A₆₀₀/C (absorbance at 600 nm per mg C mL^-1 in 0.1 M NaOH) and log(A₄₀₀/A₆₀₀) (logarithmic value of the ratio of absorbances at 400 and 600 nm). It has been found that the higher degrees of humification corresponded to the higher contents of aromatic C and N as well as carboxyl groups and the smaller contents of alkyl C, carbohydrate C, amide N, and phenolic components originated from lignin residue. However, it is unknown how long it takes to increase the degree of humification in soil. In the present study, humic acids obtained from two buried volcanic ash soils, Ts-8 (1645±29 years before present (yBP); Niigata Prefecture, Japan) and Yu-2 (701±34 yBP; Shizuoka Prefecture, Japan), were fractionated into seven or eight sub-fractions with different degrees of humification, and their ¹⁴C ages were determined. Step-wise precipitation method with acetone–0.01 M NaOH solutions was used for fractionation. Humic acids precipitated at a lower acetone concentration showed a higher degree of humification. The ¹⁴C age was also older for the sub-fractions precipitated at lower acetone concentrations. Significant correlations between the ¹⁴C age and A₆₀₀/C or log(A₄₀₀/A₆₀₀) were observed in each soil, indicating that the degree of humification increased with the passage of time and that the humic acids with higher degrees of humification were more resistant in soil. The ¹⁴C age of the Ts-8 humic acid sub-fractions ranged between 1321-1745 yBP, and that of the Yu-2 humic acid sub-fractions ranged between 524-914 yBP. Since the degree of humification of those sub-fractions covered over the range of the reported values for humic acids in various soils, it was estimated to take for 400 years to reach the most stable structure. Although ¹⁴C age differed largely between the two soils, years required to increase the degree of humification were similar to each other. These findings suggested the possibility of estimating the mean age and stability of humic acids based on the degree of humification.