As a micro mechanical machining process, micro ultrasonic machining (micro USM) has the major advantage of producing micro-scale components or features in brittle (glass, quartz crystal, and sapphire) and hard (ceramics) materials. Micro USM is used to generate micro holes with 5μm in diameter and 3D micro cavities. However, the relationship of machining parameters such as static load, abrasive particle and amplitude of vibration and the material removal rate is not clearly understood. In this paper, a mathematical model is developed to describe the material removal process in micro USM. Experiments were carried out to verify the model. It was found that the machining speed decreases when the load is over a certain value, which is different from that of theoretical model. To understand this phenomenon, a simple model was proposed to analyze it qualitatively. It was found that the debris accumulation around the crater in a short time is the main reason resulting in the low machining efficiency.