The ultra-high strength steel AerMet100was fabricated by laser melting deposition(LMD)process.The effect of hot isostatic pressing(HIP)on high-cycle fatigue properties of the LMD AerMet100steel was investigated,and the influence of defects on fatigue behavior was discussed.Results showed that the LMD AerMet100steel had fine directionally solidified cellular-dendrite structure and coarse columnar prior austenite grains.Metallurgical defects such as gas pore and lack-of-fusion porosity were produced during the laser deposition process.After HIP treatment,the number and size of metallurgical defects had remarkably decreased.Moreover,high-cycle fatigue properties of the alloys after HIP treatment were superior to the as-deposited alloys. The ultra-high strength steel AerMet 100 was fabricated by laser melting deposition (LMD) process. The effect of hot isostatic pressing (HIP) on high-cycle fatigue properties of the LMD AerMet 100steel was investigated, and the influence of defects on fatigue behavior was discussed. Results showed that the LMD AerMet 100steel had fine directionally solidified cellular-dendrite structure and coarse columnar prior austenite grains. Metallurgical defects such as gas pore and lack-of-fusion porosity were produced during the laser deposition process. After HIP treatment, the number and size of metallurgical defects had remarkably decreased. Moreover, high-cycle fatigue properties of the alloys after HIP treatment were superior to the as-deposited alloys.