The fourth transport equation for "roughness amplification" factor An which depends on equivalent sand grain roughness height k(s), has been combined with the original three-equation k-omega-gamma, transition model. According to linear stability theory, the effective length scale of original k-omega-gamma model is amplified through An which could enhance the 1st mode time scale and lead to earlier transition. The new model is calibrated and validated by several cases with available experimental data, including flat plate, some wind turbine airfoils with different patterns of distributed surface roughness. After careful comparisons with the measurements, the new four-equation transition model performs very well and satisfactory results have been achieved. (C) 2018 Elsevier Ltd. All rights reserved.