The main aim of this study is to enhance the thermal performance of loop heat pipe (LHP) charged with nanofluid as the working fluid. Thus, experiments are conducted to investigate heat transfer characteristics of using diamond-H2O nanofluid with nanoparticle mass concentration ranged from 0% to 3% in a LHP as a working medium for heat input range from 20W to 60W. The three-dimensional model, laminar flow and heat transfer governing equations are solved using the finite volume method. The simulations are carried out with three-dimensional model based on the characterization of the working fluid inside the LHP to give an insight into the heat transfer and fluid flow mechanism. The LHP performance is evaluated in terms of temperature distributions and total thermal resistance of LHP. It is inferred that the temperatures obtained at all points in evaporator side of LHP charged with diamond-H2O nanofluid are lower and reach their steady state faster than LHP charged with pure water. At the constant heat input, test results showed the average decrease of 5.7%-10.8% at nanoparticle mass concentrations ranging from 0.5% to 3% in R-th of LHP as compared with pure water (0%).