The flat-plate pulsating heat pipe (FP-PHP) offers several advantages, including a simple structure, low cost, high reliability, and effective heat transfer performance. These attributes contribute to its widespread use in the heat dissipation of various electronic devices. The operation of a FP-PHP involves a complex process of phase change heat transfer. To gain a clearer understanding of how the working fluid behaves within the FP-PHP, this paper presents a visualization experiment focused on FP-PHP with rectangular cross-section channels. The experiment primarily examines the start-up and heat transfer characteristics of the FP-PHP under different heating powers and inclination angles. Additionally, it analyzes the vapor-liquid two-phase flow patterns within the pipe under various operating conditions. Experimental results indicate that as heating power increases, the flow pattern in the evaporation section of the FP-PHP changes from a plug-shaped flow to a local annular flow, and subsequently from a local annular flow to a global annular flow. Higher heating power results in a shorter start-up time and a higher start-up temperature for the FP-PHP. The heat transfer characteristics are optimal when the inclination angle is at 90°. At this angle, with a heating power of 120 W, the thermal resistance of the FP-PHP measures only 0.51°C/W.