We have investigated the morphology of symmetric poly(styrene-b-methyl methacrylate) (PS-b-PMMA) diblock copolymers in narrow trenches by performing Monte Carlo (MC) simulations. We considered two types of interactions between the PMMA-attractive trench walls and the PS-b-PMMA. First, an exponential-type wall potential was applied to the PMMA segments to attract them to the wall, which has been widely used for the coarse-grained MC model. In the second case, only the PMMA blocks initially located near the trench wall were adsorbed to the wall. In both cases, the number of the PMMA lamellae was step-wisely increased each time the trench was widened by similar to 1.0L(0) (L-0: lamellae period in the bulk). However, due to its considerably thin PMMA layer, the trench width in the second case had similar to 0.7L(0) offset from that in the first case. These results imply the importance of interfacial characterization between the diblock copolymers and the trench walls. (C) 2016 The Japan Society of Applied Physics