Although intensive efforts have so far been exerted to develop laser 3D printing processes to fabricate various metal/alloy parts with good mechanical properties, control of the thermal condition in metal 3D printing still relies on case-dependent trial and error, without systematic understanding of the optimal thermal condition. In this work, the optimal thermal condition to minimize generation of various defects in 3D printing of stainless steel (SUS316L) and titanium alloy (Ti6Al4V) structures by selective laser melting was examined mainly by numerical simulation. In the first part of the study, the thermal condition and corresponding physical mechanisms for generating various defects, including the lack-of-fusion defect, keyhole-induced defect, and balling, were analyzed by varying such process parameters as laser power, scan speed, and spot size. Based on the results, general guidelines applicable to a broader range of materials were proposed in terms of the dimensionless power density (normalized enthalpy) and dimensionless heating time (normalized thermal penetration depth). In the second part, the characteristics of metal 3D printing with preheating and/or remelting the sample were inspected as well. In particular, the effects of preheating and remelting on defect generation in 3D printing were explored under different conditions and compared with those of simply changing the incident laser power and scan speed. It was shown that preheating and remelting could either decrease or increase the chance of defect formation depending on the thermal condition. Well-controlled surface remelting could decrease the surface roughness of 3D printed structures significantly. Finally, the effect of thermal condition on the mechanical properties of typical metal samples was discussed based on the comparison of the results of our numerical simulation with existing experimental data.