Void fraction is one of the most important parameters that affect two-phase flow heat transfer and pressure drop. In this paper, a commercial gamma densitometer and a high-speed X-ray radiography system developed at the University of Michigan (UM) are used to measure the void fraction in two-phase boiling flows, with water as the working fluid, in a tubular test section. The test section is made of Incoloy 800H/HT with a total length of 1.589 m, an inner diameter of 12.95 mm, and a wall thickness of 3.05 mm. These two instrumentation systems are installed on a traversing platform that travels along the vertical test section for measurements at multiple elevations. Subcooled flow boiling and natural convection boiling experiments are performed to measure the void fraction in the test section. Flow visualization images are obtained for bubbly and slug flows from the X-ray radiography system. The wall temperature of the test section is measured at 17 elevations by thermocouples. In addition to the experiments, a multiphase computational fluid dynamics (MCFD) model is developed using ANSYS FLUENT to simulate the subcooled flow boiling. The measured wall temperature and void fraction from the experiments are compared with the MCFD simulation results. The RMS (Root mean square) errors are 3.6% and 16.1% for the wall temperature and void fraction, respectively, between the experimental data and MCFD simulations.