An experimental study was performed to investigate the influence of the water based CuO nanofluid on both steady and transient operation characteristics of a small capillary pumped loop (CPL) with mesh wicked construction. The nanofluid was composed of deionized water and CuO nanoparticles with an average diameter of 50 nm. The experiment results of the transient operation indicated that both the startup time and evaporator wall temperature were reduced and the operation characteristics of the CPL were improved after substituting the nanofluid for water as the working fluid. With the increase of the nanoparticles mass concentration, the startup time of the CPL also was decreased gradually. Meantime, both the minimum startup power and the maximum operation power of the CPL had no meaningful changes. By substituting the nanofluid for water as the working fluid can not only effectively enhance the thermal characteristics of the CPL under the steady operation process, but also promote the startup performance. (C) 2010 Elsevier Ltd. All rights reserved.