Powder-based electron beam additive manufacturing (EBAM) is a type of additive manufacturing (AM) that utilizes an electron beam to sequentially melt cross-sections of the desired part in a bed of metal powder. There is currently very little understanding of the thermal characteristics of the EBAM process. Therefore, the ability to accurately measure process temperatures is necessary before process models can be validated and closed-loop feedback control systems can be developed. Knowledge of the cooling rates experienced by the part during fabrication is also essential for the development of predictive microstructure models. In this study, a near-infrared (NIR) thermal imager was used to measure the part temperature during the EBAM process in an Arcam S12 EBAM machine using Ti-6Al-4V powder. The temperature images collected were post-processed to analyze the process temperatures along electron beam scanning path and the size of the molten pool. Several experiments of different settings were conducted to evaluate the effects of transmission loss due to glass metallization and the effects of process parameters and part overhang geometry on temperature distributions and melt-pool sizes. The major findings are summarized as follows. (1) Metallization on the glass may significantly reduce the transmission rate, and thus, measurement quality. (2) In general, the maximum temperatures during EBAM with Ti-6Al-4V powder are in the range of 2400 °C to 2800 °C, and the length and width of molten pools are in the range of 1.5 to 3.5 mm and 0.6 to 1.0 mm, respectively. (3) The beam speed and current decrease with the build height, but the decreasing rate becomes much smaller once the build height reaches about 15 to 20 mm. (4) The larger the speed function (SF), the higher the beam speed, and the smaller the molten-pool size, e.g., for length, 2.4 mm for SF20 vs. 1.25 mm for SF65 at a build height of 6.35 mm. (5) In building an overhang feature, the heat dissipation on the overhang side is much poorer due to the low thermal conductivity of the powder. However, such an effect only dominates during the building the first few layers of the overhang feature.