The current study focuses on the numerical investigation of different gases as heat transfer fluids (HTF) in parabolic trough solar collectors (PTSC) under laminar flow conditions. The examined gases are helium, nitrogen, oxygen, carbon dioxide, hydrogen, and air. The governing equations such as continuity equation, Navier-Stokes's equation, and energy equation has been discretized using the finite difference method (FDM). The results obtained using the in-house developed computer code were compared to the data available in the literature and found to be remarkably similar. The analysis has been done to find out the best suitable HTF from these gases and found that carbon dioxide and air have the highest capability of enhancing thermal performance compared with other gases considered in this study. The air has been selected as HTF over the carbon dioxide because of environmental issues, etc. The effect of various parameters such as inlet temperature of HTF, Reynolds number, and solar flux on the temperature distribution and Nusselt number have been investigated.