Surface quality of workpiece during micro-grinding is an ever increasing concern in manufacturing industries at present. Micro-grinding offers a high potential when machining micro-structures in hard and brittle materials. In this paper, the authors propose a new analytical roughness model that enables to analyze the generation of surface; the model is developed on the basis of the number of effective abrasives and the single abrasive depth of cutting into the workpiece. The authors now initiate an experimental investigation to establish the model. To reveal the mechanism of the formation of surface in hard and brittle materials and validate the accuracy and science of the proposed model, an orthogonal micro-grinding experiment on sapphire and silicon has been designed and conducted. With this new analytical roughness model, the roughness value of the micro-slot could be predicted. In this experiment, we use 200# tool to machine the micro-slot, so the surface quality is not good. A simple relationship between surface roughness and chip thickness was obtained, which was validated by the experimental results of silicon and sapphire micro-slot grinding. Measurements of process forces, surface quality, and accuracy were carried out, and the results are discussed.