AbstractA better understanding of the natural regeneration in tropical forests could help develop more effective restoration strategies. This study examined relationships in structural and diversity attributes of natural regeneration in five forest fallow ages after slash‐and‐burn agriculture (6 months, 2 years, 5 years, 10 years, and 15 years) in the Republic of Congo. For each fallow age, all stems with a diameter ≥1 cm and below 5 cm (height ≥ 130 cm), corresponding to natural forest regeneration, were identified to the species level, and measured (diameter and height) in 12 plots (10 m × 10 m). Three structural attributes (stem density, maximum diameter, and maximum height) and five diversity indices (species richness, Shannon diversity, Simpson diversity, Fisher's alpha and Pielou's evenness) were estimated at the plot level. Our results revealed an increase in structural and diversity attributes with fallow ages, except stem density. The young fallow (6 months, 2 years and 5 years) had greater stem density (61 ± 48 stems), while the old fallow (10 years and 15 years) had higher maximum height (5.39 ± 2.36 m) and Shannon's index (0.94 ± 0.18). These results provided new insights into natural regeneration between young and old secondary forests.