Micro-grinding is one of the methods to create submillimeter-structures, and it has been applied to many processes such as finishing of internal surface of fuel injector valves and dental technique. Micro-grinding has recently been required to improve ground surface roughness. Slant feed grinding is a machining method to improve finished surfaces, but the mechanism of improving ground surface roughness of slant feed grinding has not been completely clarified yet. This paper proposed a theoretical method for calculating ground surface roughness of slant feed grinding by using a statistical approach. This method can clarify the mechanism, and it can also optimize conditions of slant feed grinding. A self-developed micro-grinding device was then employed to confirm the calculation method experimentally, and the experimental results agreed with the theoretical calculation.