This paper aims at presenting the heat transfer analysis of flow boiling in horizontal smooth and micro-finned tubes through visualization in diabatic conditions. Visualization of R410A flow boiling experiments were conducted in a transparent concentric heat exchanger. 3D printed clear resin tubes with different geometries are placed in an outer glass tube with annular water flow for visualizing the flow during evaporation. Parameters of generated bubble such as departure diameter and frequency are quantified based on visualization using high-speed camera and video processing techniques. Heat transfer in the test section is analyzed through the temperature measurements and estimated bubble parameters. Two regions of incipience of boiling are found in the current experiments. As the superheat is insufficient to reach the onset point of nucleate boiling, the heat transfer is governed by forced convection. At a higher heat flux, the heat transfer is by the mechanisms of the forced convection and surface nucleate boiling.