The heat transfer performance of oscillating heat pipes (OHPs) with graphene nanoplatelet (GNP) nanofluids was investigated experimentally. In these experiments, the GNP nanofluid concentrations were 1.2, 2.0, 5.7, 9.1,13.8, and 16.7 vol%, the heating power ranged from 10 to 100 W, and the filling ratios were 45%, 55%, 62%, 70%, and 90%, The results indicate that the heat transfer performance of OHPs is improved by using GNP nanofluids as the working fluid compared to an OHP with deionized water (DW). At appropriate filling ratios (55%, 62%, and 70%), the optimum range of GNP nanofluid concentrations was 2.0-13.8 vol%. The maximum reduction in thermal resistance was 83.6% for an OHP filled with a 2.0 vol% GNP nanofluid compared to the DW OHP at the same filling ratio (62%) and heating power (80 W). At a low filling ratio (45%), adding GNP nanofluids to DW could alleviate dry-out and improve the heat transfer performance of the OHP. (C) 2018 Elsevier B.V. All rights reserved.