Effects of surface roughness on the transition of flat-plate boundary layers under a high-lift airfoil pressure gradient with low incoming freestream turbulence level have been investigated. Time-resolved streamwise and wall-normal velocity fields with surface roughness values of Ra/C = 0.065·10−5, 4.417·10−5 and 7.428·10−5 have been measured at a fixed Reynolds number of 5.2·105 and freestream turbulence intensity of 0.2%. For the reference Smooth surface of Ra/C = 0.065·10−5, a laminar separation bubble forms from about 64% to 83% of the chord length. Displacement thickness increases downstream of separation and then decreases during the transition (reattachment), and momentum thickness increases due to the vortices shed from the separation bubble. Increasing surface roughness has little impact on the laminar boundary layer separation onset but reduces the height and length of the separation bubble and induces earlier transition. For Ra/C = 4.417·10−5, displacement thickness during transition is slightly thinner and the overall momentum deficit is slightly lower than those for Ra/C = 0.065·10−5. For Ra/C = 7.428·10−5, the separation bubble becomes hardly visible as the transition mode approaches the attached mode, and turbulent mixing by the wall-bounded turbulence becomes dominant. In addition, the portion of turbulent wetted area increases due to earlier transition, and momentum deficit increases more rapidly in the turbulent wetted area. Thus, the overall momentum deficit for Ra/C = 7.428·10−5 is larger than that for Ra/C = 0.065·10−5.