Abstract: Pinus massoniana, Cunninghamia lanceolata, and Phyllostachys edulis are the main afforesting species in the low hilly areas and valley of the southern Yangtze River for commercial purpose. Ecological problems such as the monotonous community structure and low biodiversity are common in these plantations. To understand the effect of forest harvesting gap on the understory plant diversity in the three plantations, different size gaps with identical habitats were conducted with cano-pies as control in the three plantations in Baiyun Mountain. The life form spectrum of plants and plant diversity were compared among the different forest gaps. The results showed that species richness was the highest in C. lanceolata plantation (57 species), followed by P. edulis plantation (53 species) and P. massoniana plantation (41 species) in two years after harvesting. In the early period, plant species were more abundant in the gaps than that in the controls in C. lanceolata and P. edulis plantations, but there was no significant difference between the gaps and the control in P. masso-niana plantation. The percentage of phanerophyte plants were significantly higher in the forest gaps covered 500 m² area than that with other sizes. The percentage of the hemicryptophyte and therophyte plants were higher in the forest gaps covered 250 m² area. The percentage of geophyte plants were higher in the forest gaps with 50 m² area than the others. The resembling coefficients between the different size gaps were higher than that between the gaps and the control in the same plantation. The resembling coefficients between different size gaps in the same plantation were higher than that between different size gaps in different plantations. The resembling coefficients between the gaps and the control in different plantations were generally lower than the others. The species richness, Simpson ecological dominance index, and Shannon diversity index differed significantly for different size gaps. Except for Simpson ecological dominance index, the indices were significantly influenced by the plantation types. The effects of the interaction between plantation types and gap sizes on these indices were not statistically significant. Forest harvesting gaps altered life form spectrum of understory plants and increased plant diversity in the three plantations