Laser Additive Manufacturing (LAM) is an emerging technique that facilitates the creation of intricate, customised 3D components directly from computer design. This method not only holds the potential to transform manufacturing processes, but it also enables unprecedented microstructure and mechanical properties in many metals. However, its highly dynamic temperature field and multiple thermal cycles usually induces microstructural heterogeneity in the as-fabricated samples. Hence, it is critical to establish an in-depth understanding of the correlation between the temperature field and resultant microstructure in LAM. Synchrotron X-ray radiography offers a unique capability to monitor the LAM process in real-time. This technique has already been used to study the laser-powder interaction, Marangoni flow characteristics and defect formation. In this case study, we further expand the application of in situ X-ray imaging to characterize the temperature field through collecting data on the melt pool geometry of a Ti-8.5Cu alloy during the LAM process. The melt pool geometry in each layer is used to calibrate the laser energy distribution and heat transfer parameters in the Finite Element (FE) model to simulate temperature evolution. The cooling rate extracted from the simulated temperature field provides critical information to understand the different microstructures in each layer, reflected by different volume fraction of pearlite versus martensite as well as the different interlamellar spacing inside pearlite nodules along the build direction. The effect of cooling rate and thermal gradient on the parent grain size in different layers is also discussed.