In this paper, thermal performance of the diatomite-deionized water nanofluid usage as the working fluid in a wickless loop heat pipe at varying operating conditions was experimentally and numerically investigated. A straight copper tube with an inner diameter of 13 mm, which is of 1 mm wall thickness and 1 m in-length, was used as the thermosyphon in the experimental work. The pipe was charged with deionized water and diatomite-based nanofluid respectively, by one-third of the overall volume of the heat pipe. Diatomite has high absorption capability and thus it can be considerably absorbed by the deionized water. Herewith, the disadvantages of the nanofluids, flocculation and sedimentation, are remarkably annihilated. Moreover, by using a surface-active agent, Triton X-100, these disadvantages were tried to be removed. The experiments were conducted for varying heat loads in evaporator section and cooling water mass flow rates in condenser section of the heat pipe in order to determine the operating parameters under maximum efficiency conditions of the heat pipe. Using experimental findings such performance parameters as efficiency, thermal conductivity, improvement rates in heat transfer were determined and then numerical analysis were performed. The maximum efficiency and the improvement in thermal resistance of the heat pipe when it is filled up with diatomite-based nanofluid in place of deionized water were obtained from the recurrent tests as 14.57% and 43.4%, respectively. Consequently, the experimental and numerical findings were compared to each other and it is pointed out that they are in a good agreement.