This Letter reports on the results of experiments aimed at obtaining high-quality super-smooth surfaces by investigating the laser polishing of fused silica. A maximum reduction in root-mean-square roughness to 0.156 nm was achieved, and laser-polished surfaces exhibited virtually no micro-defects or damage. A subsequent analysis using a multi-physics numerical model revealed the underlying physical mechanism of laser polishing of fused silica. The model simulated the surface smoothing process of laser polishing and demonstrated the effects of surface tension, the Marangoni effect, light pressure, and gravity in the process. It was found that the surface tension dominates the surface smoothing process, and it is a critical factor for achieving sub-nanometer micro-roughness of laser polishing of fused silica. Additionally, the model was successfully applied to predict the residual surface roughness of laser polishing.