The electron beam additive manufacturing (EBAM) technology builds parts layer-by-layer from metal powder using an electron beam. EBAM is capable of producing fully melted metallic parts with fine microstructures and superior mechanical properties. However, the microstructures, determined by the process thermal history, and yet, governing the attainable mechanical properties, are very complex in EBAM. Further, there has been little systematic study of the relationship between the process parameters and the microstructures in EBAM. Therefore, understanding the microstructure evolution is vital to achieve the desired properties of the fabricated components. The primary objectives of this research are: (1) to characterize Ti-6Al-4V powder in EBAM, (2) to analyze the process parameter effects on the microstructure, (3) to model the microstructure evolution. The research approaches include: (1) characterization of the powder using metallographic method, (2) microstructural characteristics of EBAM solid parts made by various beam scanning speeds, (3) numerical modeling to conduct thermal analysis and phase field modeling to simulate microstructure evolution, and phase transformation kinetics to calculate the phase composition. The major findings are summarized as follows. (1) Preheating results in metallurgical bonds of the powder during the EBAM. The calculated porosity of the preheated powder is about 50%. (2) The specimens of the part side surfaces show columnar prior β, while the scanning surface specimens show equiaxed grains. A higher beam scanning speed leads to a smaller grain size. The width of the columnar structure decreases with the increase of the scanning speed, 109.7 μm at 214 mm/s vs. 37.1 μm at 529 mm/s. The α-lath thickness is 1.5 μm for the sample with the lowest scanning speed, while the thickness is 1.0 μm for other scanning speed samples. (3) The finite element method is able to simulate the temperature history and melt pool geometry during EBAM. The phase field model is able to simulate the morphology and solute concentration of EBAM parts. In addition, a larger beam scanning speed results in a higher percentage of martensitic structures, and the fraction of martensites is about 30% for the highest scanning speed sample compared to around 10% for the lowest scanning speed sample.