To study systematically the effects of drought on root characteristics and biomass allocation of main afforestation species at Loess Plateau, two-year-old seedlings of Pinus tabulaeformis, Robinia pseudocacia, Platycladus orientalis and Hippophae rhamnoides were treated with three soil moisture levels, i.e. 70%, 50% and 35% WHC for three treatment durations (7, 14 and 21 days). Root biomass, aboveground biomass and fine root (Φ2 mm) properties (surface area, length and tips) were measured. The results showed that: 1) As the drought intensified and treatment time prolonged, the root biomass and fine root indices of Pinus tabulaeformis increased significantly. The root biomass of Robinia pseudocacia increased significantly, but fine root indices did not. The root biomass of Platycladus orientalis increased in moderate drought but dropped in severe drought, and the two levels had a tendency to decrease as treatment time elongated, and fine root index reduced significantly only in severe drought (21 days). The root biomass and fine root index of Hippophae rhamnoides increased markedly in moderate drought condition but the increment decreased for longer treatment time and the seedlings could not survive under severe drought conditions. 2) The mass allocation pattern of roots, stem and leaves had changed in drought, and there was a significant difference among the four tree species.