Abstract: The morphologies of silicon surfaces are modified with the single Nd:YAG nanosecond laser pulse (wavelength 532 nm) in air. The influence of single pulse laser energy density and spot size on the morphology is studied. The field emission scanning electron microscope (FESEM) and the atomic force microscope (AFM) measurement results show the formation of completely different morphologies in different conditions, and the thermodynamic process of nanosecond laser irradiating silicon is analyzed. We find that when the energy density is close to the melting threshold of silicon, and the spot size is as small as 8 m, the spike structures are fabricated. While increasing the energy density and the spot size, the conical spike structures gradually disappear, and the profile of the irradiated surface shows swellings and craters. We obtain approximate analytical solutions for the morphology by the hydrodynamic model, and the theoretical data show good agreement with AFM data for microstructures. The reason of the microstructure formation is that surface tension force drives the molten silicon to flow. The surface tension depends on the surface temperature gradient and the surfactant concentration. With the total effects of thermocapillary by the surface temperature gradient and surfactant compositional capillarity, the spike, swelling and craters can be fabricated.