On investigations of fishery resources during 2008-2009, the characteristics of the structures of fish assemblages from 20 lakes in Songnen Plain were analyzed. From total of 67249 fish individuals and 6773.6 kg fish weight collected in 20 lakes during 2008-2009, we found 44 species of 34 genus, 9 families, 4 orders, which included 107 surveys by gillnet, trawl net and pull net. The index of dominant degree were over 10000 by 21 species that formed-sized fishes by 9 species and small-sized fishes by 12 species. Dominant species were 17 species that formed large-sized fishes by 8 species and small-sized fishes by 9 species. Margalef index (the d_Ma value) were 0.908-3.096 (1.615±0.615), Simpson index (λ value) were 0.090-0.568 (0.222±0.110), Gini index (the D value) were 0.432-0.910 (0.778±0.107), Shannon-Wiener index (the H value) were 1.012-2.690 (1-878±0.431), and Pielou index (the J value) were 0.407-0.915 (0.777±0.107), The correlation coefficients (λ value) a(the d_Ma value with the A value, the D value, the H value and the J value were -0.670, 0.665, 0.867 and 0.340, respectively, the λ value between the D value, the H value and the J value were -0.631, -0.924 and-0.896, respectively, the D value with the H value and the J value were 0.924 and 0.896, respectively, and the H value with the J value was -0.639 (P<0.01). The structures of fish assemblages from 20 lakes are not stable overall.