In this paper, the heat transfer characteristics of a thermal flow sensor are investigated experimentally and numerically. Deionized water (DI-water) is employed as the working fluid. Operation mode with a constant heater temperature is considered in our experiment. The main part of the thermal flow sensor is a cylindrical copper heater. A Laser Induced Fluorescence (LIF) method is used to measure the full temperature field of the fluid in a microchannel. A specific number of flows were studied to allow investigation of the temperature distribution in a microchannel. The flow direction and velocity can be predicted based on the temperature distribution. A numerical simulation of conjugate forced convection- conduction heat transfer has been employed to investigate the predicted heat transfer processes in the thermal flow sensor. The measured temperature profiles along the central axis of the microchannel and the temperature differences between two positions upstream and downstream at different flow rates were compared with the numerical simulation results. Since the simulation and experimental measurements agree, the results show that the LIF method is suitable for temperature characterization on a microscale and confirms satisfactory characterization of the thermal flow sensor utilized here.