In the course of our screening program, the EtOAc extract of an Aspergillus sp. (strain M512), from marine sediment collected in the sea of Co To-Thanh Lan (Quang Ninh, Viet Nam), exhibited antimicrobial activities against three gram positive bacteria (Enterococcus faecalis ATCC29212, Stapphylococcus aureus ATCC25923, Bacillus cereus ATCC14579), and one yeast strain Candida albicans ATCC10231. This paper reported the isolation and structural elucidation of seven secondary metabolites including two alkaloids, 3,4-dihydroxy-6,7-dimethyl-quinoline-2-carboxylic (1), norharman (2) and five cyclodipeptides, cyclo-(Leu-trans-4-hydroxy-Pro) (3), cyclo-(Phe-trans-4-hydroxy-Pro) (4), Cyclo-(Phe-Pro) (5), cyclo (Pro-Tyr) (6), and cyclo-(Pro-Leu) (7) from culture broth of Aspergillus sp. M512. Their structures were determined by spectroscopic analysis including IR, MS, 1D-NMR and 2D-NMR, as well as comparison with literature data. All isolated compounds were evaluated for their antimicrobial activity against a panel of clinically significant microorganisms. Except compounds 1 and 2, with MIC values ranged from 32 to 128 μg/mL, five other compounds do not show inhibition activities against tested microorganisms. © 2020, Publishing House of Natural Science and Technology, VAST. All rights reserved.