Selective Laser Sintering (SLS) is an additive manufacturing technique in which thin layers of fine powder are fused by selective sintering with a high-powered laser to create a 3D part directly based on a digital instruction file. A characteristic aspect of the process is the high cooling rate experienced by the deposited material volume, which gives rise to directional cooling and the generation of columnar microstructure. The fabrication of fine sub-millimetre features leads to the modification of heat flow patterns that in turn affects the microstructure produced. Electron Backscattered Diffraction (EBSD) mapping was performed on samples of 316 stainless steel consisting of a set of thin (sub-mm) strut structures protruding from a thick base. This sample geometry was chosen to reveal the role of the transition from a bulk base (representing a large heat sink) to a thin strut (unidirectional heat conduction) in this additive manufacturing process. EBSD data was collected from the base region, the transition region between the base and the strut, and in the strut region. This allows examining the effect of the heat flow difference between the strut region and the base region on sample microstructure. The greater temperature gradients in the strut region lead to significantly refined grain structure compared to the base region.