Sacrificial layer etching through a small circle window is studied in this work. It is found that the etching rate decreases with etching time quickly and the etching rate decreases more quickly with a smaller etching window. The existing model cannot fit the experimental data well. The error of etching rate between the existing model and the experimental data increases with etching time, which will reach as high as 50% when the etching length reaches 1000 μm. A new model is proposed to explain these phenomena by considering the diffusion coefficient of HF as a function of concentration and temperature. To simplify the model, only HF is considered in this work for the etching and diffusion process. Fortunately, error of etching length between the proposed model and the experimental data is less than 5%, which is acceptable for most case. As etching length increases further, the etching products will affect the etching and diffusion process seriously. In that case, more complicated model may be necessary for accurate prediction.