This book investigated two different thermal sensors for intravascular flow monitoring based on convective heat transfer principle. Both sensors were fabricated by MEMS (micro-electro-mechanical systems) and semiconductor processing. The first thermal flow sensor is one-dimensional (1-D) flexible shear stress sensor. Utilizing the state-of-art MEMS technology, its primary advantages were structural flexibility, bio-compatibility and small size. Then, a novel 3-D thermal flow sensor was developed based on previous flexible 1-D sensors. With computational fluid dynamics (CFD) analysis, 3-D sensor can potentially achieve a better measurement of shear stress distribution in the presence of time-varying component of blood flow and realize more accurate assessment of intravascular convective heat transfer.