This paper experimentally investigates how different mass concentration and aspect ratio multiwall carbon nanotubes (MWCNTs) acetone nanofluid affects the heat transfer performance of a flat plate heat pipe (FPHP). Different mass concentration and aspect ratio MWCNTs-acetone nanofluids are prepared without any surfactants or additives using the two-step method. Aspect ratios of MWCNTs are 666 (M1) and 200 (M2), respectively, and their according mass concentrations are 0.002, 0.005, 0.01, and 0.015 wt. %, respectively. The thermal resistance and wall temperature of the FPHP are experimentally obtained when the above-mentioned nanofluids are used as working fluid. The results showed that different mass concentration affects the heat transfer performance, therefore, there is an optimal MWCNTs-acetone nanofluid mass concentration (about 0.005wt. %). Also, the results showed that the thermal resistances of the FPHP with M1-acetone nanofluid (0.005 wt. %) and M2-acetone nanofluid (0.005 wt. %) are reduced 40% and 16%, respectively. Based on the above experimental phenomenon, this paper discusses the reasons for enhancement and decrement of heat transfer performance of the different mass concentration. For the M1-acetone nanofluid, the investigated FPHP has a thermal resistance of 0.26 °C/W and effective thermal conductivity 3212 W/m k at a heat input of 160 W. For the M2-acetone nanofluid, the investigated FPHP has a thermal resistance of 0.33 °C/W and effective thermal conductivity 2556 W/m k at a heat input of 150 W. The nanofluid FPHP investigated here provides a new approach in designing a high efficient next generation heat pipe cooling devices.