Abstract: Single-phase water flow behaviors in narrow rectangular ducts were studied theoretically and experimentally. The results show that aspect ratios have significant influence on flow resistance. The frictional resistance in rectangular duct increases as the aspect ratio decreases both in laminar flow region and the smooth region of turbulent flow. The visual observation shows that critical Reynolds number for narrow rectangular ducts ranges from 2 500 to 2 600, and the aspect ratio nearly has insignificant influence on flow regime transition. The single-phase flow resistance is influenced significantly by wetting perimeter and velocity distribution, which vary with the aspect ratio in laminar flow. For turbulent flow, the turbulent mean velocity in viscous sub-layer and wetting parameter become important parameters affecting the flow resistance.