Many upcoming designs of nuclear reactors conceptualize use of supercritical fluids (SCFs) due to certain associated advantages. Supercritical helium (SC-He), supercritical carbon-dioxide (SC-CO2), and supercritical water (SCW) find proposed applications in primary/secondary heat removal cycles in nuclear reactors. The knowledge of heat transfer characteristics of the working fluid is essential prerequisite for its actual application. Similarly, for SCF, heat transfer deterioration (HTD) is an area having much ambiguity and thus has attracted many researchers in the recent past. Still, conclusive guidelines for design applications of SCF are lacking due to dramatic variation of thermos-physical properties of SCF in the pseudo-critical region. In this paper, extensive experimental investigations are undertaken to study HTD of SC-CO2 under natural circulation (NC) system. Experiments conducted at pressures between 7.9 and 12.31 MPa for vertical heater horizontal cooler (VHHC) and vertical heater vertical cooler (VHVC) are reported. Effect of heat flux, operating pressure, and cooler orientation are discussed based on the experimental data generated. Methodologies available in literature for identification of HTD are also examined with the present experimental data. The experimental data are compared with the HTD criteria available in literature and a new criterion for onset of HTD is proposed for vertical flows under natural circulation. Role of buoyancy (Bu) and acceleration (Ac) parameters are also investigated. Noticeable HTD is observed for Bu > 3 × 10−6 whereas Ac is not found to affect the HTD in this study.