Abstract: The plate-type fuel has compact structure and narrow coolant channel. The size effect of large aspect ratio makes the narrow rectangular channel have certain heat transfer enhancement effect, and can meet the requirements of miniaturization, high power and high performance of heat exchanger. Due to the compact structure and narrow coolant flow channel, the plate-type fuel shows a high-power density and enhance heat transfer performance. Due to the fluid micro-agglomerations near the channel wall are affected by viscous shear stress and turbulent shear stress, the fluid state in the boundary layer is complicated under the singlephase condition. The two fuel plates form a 2 mm to 3 mm width narrow rectangular channel, in which the viscous and turbulent shear stress show great effects on the structure of velocity boundary layer. As a result, the flow field in the boundary layer of rectangular channel shows different characteristics as compared to the conventional normal size channels. In the present study, the visualization method was used to construct the velocity boundary layer of the narrow rectangular channel of 2 mm and 3 mm width. Under the conditions of laminar and turbulent flow, the friction resistance of the rectangular channel was measured by the differential pressure transmitters, and the instantaneous and time-averaged velocity, and vorticity distributions of the flow field were captured by the particle image velocimetry (PIV) technique. The experimental results show that the axial velocity distribution distributes parabolically in laminar flow conditions, i.e., the velocity gradient is small near the wall, while the velocity is large in the center of the rectangular channel. The friction resistance coefficient conforms overlapping with the Shah relationship. In turbulent flow conditions, the velocity distribution is steeper near the wall and relatively flat in the center of the channel. The friction resistance coefficients of 3 mm and 2 mm channels overlap with the Blasius correlation and the McAdams correlation, respectively. The thickness of velocity boundary layer in rectangular channel increases with the increase of Reynolds number, and the thickness of turbulent velocity boundary layer is greater than that of laminar velocity boundary layer at high Reynolds number. The dimensionless velocity distribution of turbulent velocity boundary layer in rectangular channel conforms to Spalding formula. The turbulent boundary layer constants in 3 mm channel and 2 mm channels are determined as κ＝0.418 7, B＝5.45 and κ＝0.394 5, B＝4.92, respectively. It is found that a Reynolds shear stress peak area exists within 0.1-0.3 mm from the narrow side wall of the rectangular channel. With the increase of Reynolds number, the thickness of the viscous bottom layer of the velocity boundary layer gradually reduces, the proportion of logarithmic law layer increases, and the Reynolds shear stress peak region approaches the wall direction. Reducing the channel width from 3 mm to 2 mm will restrict the development of velocity profile near the wall. The velocity gradient near the wall increases and the turbulence intensity reduces.