The purpose of this study is to investingate the characteristics of demersal fish assemblages in Yilan Bay. A total of 33 bottom trawl operations were conducted along the waters with various depths of A (20 ~ 50 m), B (50 ~ 100 m), C (100 ~ 200 m) and D (200 ~ 400 m) by a fishing boat “Sin-Yun-Fa 63” during November 2 - December 16, 2004. The mesh size of trawl net at cod end was 3 cm. The data were then complied to analyze their catch per unit effort (CPUE). The H’ in the area of A, B, C and D were 1.13, 1.34, 1.44 and 1.38 by using the analysis of Shannon-Wiener index (H’) method. The lowest H’ at station 22 was 0.41, but examined species was only 27. Among them, the horse mackerel (Trachurus japonicus) occupied 80.51% of the catch and was the dominant species in this station. The H’ at other stations were greater than 0.51. The relationship between the H’ and the depth layer was not significant (p<0.05). Area groups and species groups were classified by using the Bray-Curtis dissimilarity index and UPGMA method. The result showed that fishes could be classed into three areas and five species groups. Crabs could be classed into three areas and four species groups. Cephalopods could be classed into three areas and three species groups. The area groups varied in CPUE and number of species. The species groups varied in depth and CPUE.