Experiments have been conducted on a flat-plate in an open type wind-tunnel at Seoul National University to investigate effect of surface roughness on transitional boundary layers under zero pressure gradient. Two different magnitude of surface roughness have been tested; one it aerodynamically smooth (k+=0.07~0.19) and the other is transitionally rough (k+=2.71~7.05) regime. By using hot-wire anemometry mounted on 3-axis traverses, mean velocity and streamwise turbulence intensity have been measured. Results indicated that surface roughness increases the streamwise turbulence intensity at outer layer (y/δ~0.4). For this region, mean velocity defect also occurred and that increases a significant enhancement of mean flow momentum deficit.