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We have presented the mechanisms of nanosecond laser-induced specific nanostructures on a SiGe film by changing laser irradiation intensities. Our experimental results show that the types of the formed nanostructures were sensitive to the laser intensity because the structures such as cone-like protrusions, islands, and droplet-like ripples upon the irradiated SiGe film successively transform to ripple structures as the irradiation intensity decreases. Moreover, we obtain the threshold of the laser intensity for each nanostructure. Further studies give compelling evidence that the laser irradiation undergoes a transition from an initial thermal effect to an optical interference, which results in the evolution of SiGe nanostructures.