Therminol-based nanofluids with graphite nanopowders (GNs) were proposed as an efficient working fluid in direct-absorption solar collectors (DASCs), especially for elevated-temperature applications. In this work, Therminol VP-1/GN nanofluids (TG nanofluids) were prepared in two steps with surfactant (Triton X-100). In the synthesis procedure, the vortex generator for coating GN with sticky surfactant (mu = 240 mPa.s) and the ultra-sonicator for diluting GN/Triton X-100 mixture with Therminol VP-1 are used. We have demonstrated excellent dispersion stability up to 4 weeks and even after heating over 100 degrees C without any surface modification of nanoparticles. The feasibility of using the synthesized TG nanofluid as a working fluid in DASCs was experimentally demonstrated by measuring its absorption coefficient as well as by characterizing photo-thermal conversion performance. The absorption coefficient of the TG nanofluid at 0.004 wt.% of GNs was measured to be 1.12 cm(-1), which is more than eight times enhanced from that of bare Therminol VP-1 (ie, 0.13 cm(-1)) in the wavelengths from 350 to 1600 nm. Finally, the temperature increment of 43 degrees C was achieved after t = 3600 s under the 5-sun condition in the static photo-thermal conversion experiment. Moreover, the low viscosity of TG nanofluids (only 2.2% higher than that of the bare Therminol VP-1) supports the superiority of TG nanofluids to be applied as working fluid in DASCs.