An experimental study was carried out to investigate the thermal performance of solar heat pipe collector at outdoor test condition. The thermal performance of wickless heat pipe solar collector was investigated by using CNT–water nanofluid. Carbon nanotubes (CNT) nanoparticles with diameter 10–12 nm and 0.1–10 μm length were used in the present experimental investigation. The effects of various filling ratio (F.R.) (50%, 60%, and 70%) and coolant flow rate on thermal performance were discussed in this study.