Resumo. The structure dynamics of an understorey vegetation influenced by gaps caused by reduced impact logging was analyzed during 12 years in a terra firme rain forest in the municipality of Moju, Pará state, eastern Brazilian Amazon forest. Angiosperm individuals from 10 cm height up to 5 cm DBH were surveyed in a 468 m² sample area in six occasions (1998, 1999, 2000, 2001, 2007 and 2010), they were analyzed considering their life-forms (trees, shrub, lianas, herbs and palms), abundance, frequency, size classes (CT1 = 10 cm ≤ HT < 50 cm; CT2 = 50 cm ≤ HT ≤ 130 cm; CT3 = 130 cm < HT and DAP < 2 cm; and CT4 = 2 cm ≤ DAP < 5 cm) and relative natural regeneration Burseraceae, Marantaceae, Violaceae and Lecythidaceae were the most important families in the study area in every occasions, while the most representative species were Monotagma sp1., Monotagma sp2., Rinorea guianensis, Lecythis idatimon e Protium pilosum. The most frequent life forms were trees, herbs and lianas in all occasions. The dynamics of life forms depended on light intensity. The establishment of commercial and potentially commercial species was favored by gaps, suggesting that natural regeneration of those species do not need to be assisted for guaranteeing timber stocks for future harvests.