Floristic composition, stand structure, and above-ground biomass of tropical lowland rain forests were examined to compare the effects of different forest managements, i.e., old-growth forest as control, the forest harvested by the reduced-impact logging (RIL), and the forest harvested by the conventional method In Deramakot and Tangkulap Forest Reserve, Malaysian Borneo. Species diversity was rich in the old-growth forest and the forest harvested by RIL where climax and important commercial-timber species of Dipterocarpaceae dominated, while low in the forest harvested by the conventional method where pioneer species of the genus Macaranga (Euphorbiaceae) dominated. Size structure showed that Dipterocarp trees regenerated well in the old-growth forest and the forest harvested by RIL. On the other hand, Dipterocarp trees did not regenerate well in the forest harvested by the conventional method and Euphorbiaceae trees demonstrated an evidence of regeneration. Basal area and above-ground biomass in the old-growth forest and the forest harvested by RIL were higher than those of the forest harvested by the conventional method. Floristic composition, stand structure, and above-ground biomass were not different between the old-growth forest and the forest harvested by RlL. However, the species composition and above-ground biomass of the forest harvested by the conventional method were different from those of the old-growth forest and the forest harvested by RIL due to high impacts of logging. Thus, RlL management could keep species diversity, forest structure, and biomass at a pre-harvest status
