The infrared normal spectral emissivity of microstructured silicon prepared by femtosecond laser irradiation in SF 6 was measured for the wavelength range 2.5μm to 25μm. Greatly enhanced emissivity compared to that of flat silicon was observed over the entire wavelength range. For a sample with 13–14μm high spikes, the emissivity at a temperature of 100°C is approximately 0.96. The emissivity decreases slightly in the wavelength region above 8μm, but remains higher than 0.9 over most of the measured wavelength range. Also the average emissivity is less than Nextel- Velvet-811-21 Coating, it can be used stably at more wide temperatures from 100°C to 400°C. These results show the potential for microstructured silicon to be used as a flat blackbody source or silicon-based pyroelectric and microbolometer devices.