This study investigated the grain size distribution, roundness, shape, and sedimentary facies of the sediments sampled at 30 stations along Seomjin River. The surface sediments were very coarse to medium sand of -0.78 ~ 1.92Ø and well- to very-poorly sorted (0.36 ~ 2.32Ø). With respect to the interrelations among textural parameters in the sediments, mean size was highly proportional to gravel content and sorting value, and was increasingly fine downstream. The roundness of the sediments was variable in size, ranging from granule to coarse sand, and gradually changed a gradually reduced as the grains became finer. The correlations between grain size and roundnesswere r=0.46 ~ 0.76 in -2 ~ 2Ø (very angular to angular) and r=0.50 ~ 0.76 in -1 ~ 2Ø (subangular to subrounded). Synthesizing these results, we document that sandy sediments in the area are forming in the entrenched meanders that run parallel to the surrounding mountainous terrains. In particular, roundness patterns well correspond to the above-mentioned fluvial sediments. Channel and velocity changes of the rivers are closely interrelated with sedimentary facies and roundness together.