This paper presents a research about the modelling of surface roughness in the grinding process. Based on the analyzed results about the surface roughness models from the previous studies, this study was performed to develop a surface roughness model in the grinding process. The surface roughness model is proposed with two hypotheses about the shape and ratio of the number of scratches of the grinding grains leaving on the workpiece surface. The first hypothesis is that the shapes of scratches of the grinding grains leave on the workpiece surface with five types including the triangle, the curved arc, the parabolic, the semi-circular, and the curved arc of a quarter circle. The second hypothesis is that the ratios of the number of the scratches with different shapes of the grinding grains leaving on the workpiece surface are the same (all are equal to 20%). This research paper discusses about the modelling of surface roughness when considering many shapes of scratches of the grinding grains left on the machining surface. This proposed model can be used to calculate the surface roughness in grinding processes of the different pairs of grinding wheels and workpieces. The developed surface roughness model has been applied to calculate the surface roughness during the grinding of SUJ2 steel using an aluminum oxide grinding wheel. Calculated surface roughness using proposed model were quite close to the experimental results. The average difference between calculated and the experimental results was about 14.84%. This study offers a promising ability to calculate the machining surface roughness in the grinding processes.