An experimental study was carried out to investigate the thermal performance of a capillary pumped loop（CPL）using water-Cu nanofluid as the working fluid.The study was focused on the influence of the Cu nanoparticle mass fraction and the operation pressure on the heat transfer coefficients of the evaporator，the maximal heat flux and the total thermal resistance.The experimental results indicated that the thermal performance of the CPL was improved by addition of Cu nanoparticles.There existed an optimum nanoparticle mass fraction about 1.0%.The influence of operation pressure on both the evaporating heat transfer coefficient and the maximal heat flux was remarkable.The heat transfer coefficient and maximal heat flux increased with increasing operation pressure.Compared with water，the evaporating heat transfer coefficient and maximal heat flux for nanofluid could increase by 40% and 18%，respectively when the nanoparticle mass fraction was 1.0% and the operation pressure was 15.74 kPa.