We measured the fine root biomass and its depth distribution of a more than 200-year-old primitive Korean pine and broad-leaved forest and its two 20- and 80-year-old secondary Populus davidiana and Betula platyphylla forests in the Changbai Mountain, northeast China. The soil core method was used for root sampling. The results indicated that the total fine root biomass increased with stand age, ranging from 561.8 g/m2 and 674.1 g/m2 for 20-year-old and 80-year-old secondary forests, respectively, to 827.8 g/m2 for the old primitive mixed forest. Fine root biomass increased in dominant tree species but decreased in the undergrowth other woody species and herbs. Along the successional development from young to old stands, the rooting depth exponentially increased though most of fine roots were distributed in the 0-10 cm soils. In the forests, the effects of soil bulk density, soil moisture, and soil organic C and N contents on fine-root depth distribution were more significant than that of soil temperature.