Abstract The heat transfer performance of miniature loop heat pipe with graphene–water nanofluid is experimentally analysed. The miniature loop heat pipe used in the study consisted of a square flat evaporator having a size of 20 mm × 20 mm, a compensation chamber placed above the evaporator and transport lines having different diameters. The difference in diameter prevents reverse flow of vapour through liquid line and also increases the flow rate of condensed liquid through liquid line. An optimum filling ratio of 30% of the total volume of the heat pipe is used in all the experiments. The experiments are conducted for a heat load range of 20–380 W using water and graphene–water nanofluid in vertical orientation. The graphene nanosheets having 1–5 nm thickness with very low volume fractions of 0.003%, 0.006% and 0.009% are mixed with distilled water to prepare nanofluid. The experimental results indicate that the nanofluids improve the thermal performance of the miniature loop heat pipe and lower the evaporator interface temperature compared to distilled water. An optimum concentration of 0.006% provides the maximum improvement in heat transfer. The lowest thermal resistance value (0.083 K/W at 380 W) is observed for the optimum concentration and it is 21.6% below the value of distilled water. The evaporator interface temperature reached only 106.3 °C at 380 W which shows a decrease of 10.3 °C compared to distilled water. The experimental results confirm suitability of miniature loop heat pipe filled with graphene–water nanofluid for cooling applications.