Micro-grinding is a machining method to create submillimeter structures, and it can be applicable to various kinds of materials with a flexible forming ability. However, the micro tool may be easily bended by the grinding force, which results in relative low form accuracy and worse ground surface roughness. To solve these problems, ultrasonic assistance was applied to micro-grinding. This paper proposes a method to calculate ground surface roughness theoretically using statistical approach, and the ground surface improving mechanism of ultrasonic assistance was clarified. Finally, the theoretical calculation was confirmed experimentally using a developed ultrasonic-assisted micro-grinding device.