Abstract: It is found that the black silicon fabricated by femtosecond laser in the vacuum is different from that fabricated in the gas atmosphere of SF6. To study the related optical properties of this micro-structured silicon fabricated in the vacuum, the changes of its peak height and absorptance were studied by changing the energy of laser pulse. It is found that the microstructures fabricated in the vacuum can also reach the absorptance of ~95% in the spectral range of 200-2 500 nm as that fabricated in the gas atmosphere of SF6. Finally, by annealing the black silicon fabricated in two different environments, the black silicon fabricated in the vacuum has better annealing resistance. These results are very significative for the fabrication of infrared sensor.