Abstract3D printing is a rapidly growing technology for the fabrication of nickel-based alloy products in the aerospace industry because of its time efficiency, relative affordability, ease of use and design freedom. The alloy was initially built by selective laser melting technique, followed by stress relieving, hot isostatic pressing, and heat treatment processes. How process parameters affect the microstructure of 3D printed alloy 718, however, is not well understood. In this research, the grain size changes and the microstructural evolution of as-built samples were studied under different heat treatment conditions. The HT-1 treatment (solution treatment at 950 °C/1 h/water quenching followed by two-stage ageing at 720 °C/8 h/furnace cooling to 620 °C/8 h/air cooling to room temperature) reduced the grain size of aged microstructure twice compared with the standard AMS 5662 (solution treatment at 980 °C/1 h/water quenching followed by two-stage ageing at 720 °C/8 h/furnace cooling to 620 °C/8 h/air cooling to room temperature) due to the existence of acicular grain boundary 5 phase. However, the high volume fraction of y'/y" precipitates in the HT-2 (solution treatment at 1035 °C/1 h/water quenching followed by twostage ageing at 720 °C/10 h/furnace cooling to 620 °C/10 h/air cooling to room temperature) aged microstructure led to increase the microhardness value.