The effect of seed size on the physiological quality of Leucaena leucocephala (Lam.) De Wit was studied. Seeds were screened by round perfective sieves as follows: type 1 (5.5mm), type 2 (5.1mm), type 3 (4.7mm), type 4 (4.3mm), and type 5 (3.9mm). Physiological seed quality was evaluated through germination and vigor (first counting and fresh and dry matter weight). Type 2 seeds showed higher germination percentage than type 5 ones. Vigor test (first counting) results that types 2 e 3 seeds were more vigorous as compared to type 5. Seeds of types 1, 2, and 3 were more vigorous (fresh and dry matter weight) than those of types 4 and 5.