The objective of this thesis research is to improve our understanding of the flow obstacle effect on heat transfer at supercritical and high subcritical pressures by experimentally studying the effect of different obstacles on heat transfer in two vertical upward-flow test sections: a 3-rod bundle and an 8 mm ID tube. The heat transfer measurements cover the region of interest of the Canadian Super-critical Water Cooled Reactor (SCWR). A thorough analysis of the obstacle effect on supercritical heat transfer (SCHT) was performed. In the 3-rod bundle, two types of obstacles were employed: wire wraps and low-impact grid spacers. Wire wraps were found to be more effective than grid spacers to enhance the SCHT. In the tubular test section, obstacles appeared to suppress the heat transfer deterioration (HTD) or decrease its severity; obstacles also generally enhanced the SCHT both in the liquid-like and the gas- like region. The experiment in the tubular test section revealed that, at certain flow conditions (low mass flux, low inlet subcooling), flow obstacles can have an adverse impact on the SCHT. A criterion to predict the onset of this adverse effect was developed. At high subcritical pressures, obstacles increased the CHF and reduced the maximum post-CHF temperature. A comparison of the experimental data with prediction methods for the SCHT, single phase heat transfer, CHF and post-dryout heat transfer was performed. Lastly, a new correlation to predict the enhancement in SCHT due to obstacles was developed for heat transfer in the liquid-like and gas-like regions.