A simple network model is derived for describing the transient response of a two-phase gravity-assisted heat-pipe at startup, steady, and shutdown states using Al2O3 nano-fluid. In order to obtain a stable suspension of 4% Al2O3 nano-fluid aluminum oxide particles are coated with polyethlene glycol (PEG), then dispersed in a 2-hydroxyethyl methacrylate (HEMA) solution, and after that subjected to gamma irradiation at different doses. The heat pipe is divided into two heat carriers; evaporator and condenser. An energy balance for each carrier is carried out to estimate temperatures, heat transfer coefficients, thermal resistances, time constants, and other thermal characteristics at three different heat loads; 500, 1000, and 1500 W. Governing equations of the transient behavior can be simplified into first-order linear ordinary differential equations, which can be solved by the linear algebra formalism method yielding the heat-pipe temperatures. The transient response of a gravity-assisted heat pipe is found to depend mainly on the average evaporator thermal resistance. Increasing the heat loads causes a sharp reduction in the thermal resistance and in the time constants, which leads to better performance of the heat pipe. The evaporator and condenser heat transfer coefficients are thus found to increase with increasing power, so that at 500 W heat load the obtained evaporator heat transfer coefficient at steady state is approximately 100 W/m(2) degrees C, while doubling the heat load causes he to increase to as much as 1100 W/m(2) degrees C; i.e. more than 10 times the value attained with half that load. Increasing the heat load further to 1500 W gives about 3900 W/m(2) degrees C; i.e. more than 3 times the value obtained with 1000 W. The variation in the corresponding values of the condenser heat transfer coefficients are much less than those obtained for the evaporator as they lie in the range of 1534-1557 W/m(2) degrees C at heat loads 500 up to 1500 W. The effect of adding 4 vol.% Al2O3 nanoparticles to water is investigated by estimating the physical properties of the nanofluid at steady state for the three investigated input powers and comparing them with those of water. The most important feature of them is the thermal conductivity, which is found to increase by 27% above that of water. The dynamic viscosity of the 4 vol.% Al2O3 nano-fluid is found to increase by 10% over all selected powers. The replacement of ordinary cooling fluid with nano-fluid has thus no noticeable effect on the flow dynamics but it considerably enhances the heat transfer properties. (C) 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.