Selective Laser Melting (SLM) of a high strength low alloy steel HY100 is considered in the present investigation. The current work describes (i) optimization of SLM process parameters for producing fully dense parts in HY100 steel and (ii) the effects of post-processing heat treatment on the microstructure and mechanical properties. Samples have been fabricated by SLM using different combinations of laser power, laser scan speed, and hatch spacing. Fully dense samples were achieved at an energy density of 65 J/mm3. Microstructures of the as-built and heat treated samples were investigated using optical and scanning electron microscopes, X-ray diffraction, and electron backscattered diffraction techniques. The as-built sample showed fully martensitic microstructure with alternate bands of untempered (hard) and auto-tempered (soft) regions. The as-built parts are unsuitable for direct application due to untempered, hard and brittle martensite microstructure. The as-built parts were subjected to post-processing heat treatments (“direct temper” and “quench and temper”). The direct tempered samples exhibited higher yield strength and ultimate strength than the quench and temper ones. Noticeable amounts of anisotropy with respect to the build orientation, especially in tensile elongation, were observed in the direct tempered samples due to in-homogenous microstructure. Quench and temper treatment of the parts resulted in recrystallized grains with uniform microstructure. The current investigation shows that quench and temper at 650 °C is an optimum post processing treatment for HY100 SLM parts as it manifests desired strength with good tensile elongation. © 2016 Elsevier B.V.