This study experimentally investigates the effect of the cross-sectional area of vapor path on the heat transfer performance of a water-filled heat pipe with a polymer insert for optimizing its design. The thermal resistance and the heat transfer coefficient of the heat pipe with a screen mesh wick were measured at a saturation pressure ranging from 6.0 kPa to 12.5 kPa. It is observed that the changes of the capillary limit and the overall heat transfer coefficient come from the reduction of the vapor space. When the cross-sectional area of the vapor path is reduced to 48.3%, the capillary limit of the heat pipe is decreased by 22.9%. But the overall heat transfer coefficient of the heat pipe is slightly decreased by 3-7%. When the cross-sectional area of the vapor path is reduced to 76.8%, the capillary limit and the heat transfer coefficient of the heat pipe are decreased by 40.7% and 21.0%, respectively. Therefore, the reduction of the overall heat transfer coefficient of the heat pipe has no great effects according to the cross-sectional area of the vapor path. The experimental results suggest the direction of the optimization of the heat pipe in terms of space management for compact devices. Or, if there is enough margin in capillary limit, the optimized compact vapor path without losing heat transfer performance too much can be acquired or excess space can be used for special applications such as neutron absorber in nuclear control rods and structural supports in the electronic cooling for compactness. (C) 2015 Elsevier Ltd. All rights reserved.