<p>Ultrasonic vibration–assisted diamond wire sawing (UADWS) is a high-efficiency process for precision cutting monocrystalline silicon (mono-Si). The surface roughness <i>Ra</i> of slice serves as a critical parameter for evaluating machining quality. Investigating the influence of process parameters on <i>Ra</i> is essential for optimizing the sawing process. In this paper, orthogonal experiments and response surface methodology (RSM) were employed to investigate the effects of feed speed, wire speed, ultrasonic amplitude, and ultrasonic application position on surface roughness <i>Ra</i>. Experimental results show that the surface morphology of slices improves with ultrasonic vibration application. The range and variance analysis of surface roughness indicated that feed speed exerted the most significant influence on <i>Ra</i>, followed by wire speed and ultrasonic amplitude. Within the selected experimental parameter range, however, the effect of ultrasonic application position was statistically insignificant. Based on the experimental data, the regression prediction model of <i>Ra</i> was established. The maximum relative error between the predicted value and the experimental value was 7.51%, indicating that the model was effective. The RSM further revealed the interaction between process parameters: at low feed speed, the change of wire speed has little effect on <i>Ra</i>, while at high feed speed, the increase of wire speed can significantly reduce <i>Ra</i>. High wire speed will inhibit the effect of ultrasonic vibration–assisted cutting. RSM further revealed the interactions between process parameters: At low feed speed, changes in wire speed had minimal effect on <i>Ra</i>, while at high feed speed, increasing wire speed significantly reduced <i>Ra</i>. High wire speed suppressed the beneficial effect of ultrasonic vibration. This study provides experimental references for the optimization of UADWS process.</p>