Abstract: Soil drilling method was used to study fine root biomass of vegetation under different enclosure ages in limestone area. The results showed fine root biomass was 6.6287 t/hm2 in the stands after 5 years爷enclosure, 2.3063 t/hm2 after 10 years爷enclosure, 3.1316 t/hm2 after 20 years爷enclosure and 2.8316 t/hm2 after 30 years爷enclosure. It declined with the increase of soil depth in each enclosure types and showed significantly higher value in 0~10 cm depth than the other two layers. Annual dynamic of fine root biomass under different enclosure years followed a bimodal pattern.