In this thesis, we numerically analyze heat transfer in selective laser sintering (SLS) process to study the temperature distribution in the powder bed and its effective parameters. To do so, we first analyze heat transfer in powder within a rather simple geometry to find the most appropriate approach for the simulation of heat transfer in powder. Specifically, the powder is heated in an aluminum platform inside an oven and then cooled under air free convection. We then simulate this process by modeling the powder from two perspectives: porous media and non-porous media. The simulations are carried out under specific boundary conditions with transient solutions. The results of both models are then compared with the test results to choose the perspective that is more consistent with the reality. When the choice is made, we start analyzing heat transfer in the SLS process by a 3-dimensional approach. We mesh the whole encasement including the powder and the air inside the encasement, while taking into account some details of the encasement components. Under specific boundary conditions, we simulate the SLS preheat process from ambient temperature to near melting temperature of the powder, by considering all types of heat transfer i.e., conduction, convection, and radiation. This leads to the calculation of the distribution on the surface and inside the powder bed. We further verify the simulation results with the experimental results and the thermal images of the powder surface. Based on the verified simulations, we carry out a parametric study to find the effective parameters that can lead to better temperature distribution in the powder bed. Specifically, we look into the effect of encasement’s gas type, the change in gas pressure, and the change in the gas specific heat, on the final powder bed temperature distribution. Keywords: selective laser sintering (SLS), preheat process, numerical simulation, heat transfer in powder, powder temperature distribution