It is difficult to avoid the development of nano inclusions by crucible erosion during gas atomization process. Selective laser melting equipment was used to print 18Ni300 die steel powder with nanoalumina powder. The effects of nanoalumina powder additions on microstructure and mechanical properties of 3D-printed samples were studied to determine the tolerance limit for atomization pulverization. When the nanoalumina addition (mass fraction) is less than 1%, the second phase segregation is not detected for printed or low-temperature annealed samples. When more than 1% nanoalumina is added, the Ni₃(Al, Ti) and Al₂O₃ segregation phases appear, and the number of internal cracks and spherical pores increases, decreasing in mechanical properties. The annealed sample containing 1% nanoalumina has the maximum tensile strength of 1658 MPa.