Abstract: To reveal the heat transfer mechanism and cooling effect of inter-wrapper flow (IWF) under natural circulation conditions in a sodium-cooled fast reactor (SFR), 30 sets of steady-state natural circulation experiments were conducted on the CFR-CORENa local core sodium test facility, jointly developed by the China Institute of Atomic Energy and Tsinghua University. The test section comprised 23 subassembly simulators with inter-wrapper flow channels, forming a partial core mockup with 5 heatable columns. The simulators, based on a typical 169-wire-wrapped fuel subassembly, were scaled to approximately one-half of the prototype cross-section. By independently adjusting the power distribution among subassemblies (power ratios from 1∶0.4 to 1∶1.07), the opening states of external IWF source channels (core barrel bypass, peripheral inter-wrapper gaps, and lateral hot pool flow), and the cooling conditions, comprehensive thermal-hydraulic data were acquired. Measurements included flow rates by electromagnetic flowmeters and temperatures by 259 K-type thermocouples distributed over five axial planes. The results demonstrate that without external low-temperature sodium supply, the IWF operates primarily in an internal recirculation mode dominated by lateral heat redistribution (thermal balancing) among subassemblies. In this mode, a significant portion of heat transferred outward through the wrapper walls is reabsorbed by adjacent lower-power subassemblies, constituting an inter-subassembly thermal short-circuit. Consequently, the net heat removal fraction is limited and strongly controlled by the uniformity of power distribution—approximately 5% under non-uniform conditions and about 9% under relatively uniform conditions. When external source channels are opened, the IWF is continuously supplied with cold sodium and transforms into an independent cooling loop parallel to the intra-subassembly flow. The wrapper heat transfer mode shifts from thermal short-circuiting to a collaborative heat rejection path of “subassembly→IWF→external”, with all subassemblies discharging heat consistently to the same low-temperature sink. The net heat removal fraction reaches up to 46%, comparable to its flow fraction. Under sufficient external flow supply with good cooling source conditions, the heat removal capability per unit mass flow rate becomes essentially equivalent to that of the intra-subassembly flow, while under most operating conditions it ranges from 0.5 to 1.0 times that of the intra-subassembly flow. Within the investigated power range, the maximum transverse temperature difference across the fuel-region inter-wrapper channels is approximately 25 °C, and the maximum intra-subassembly cross-sectional temperature difference is approximately 27 °C. Notably, these temperature differences do not increase significantly with rising absolute power, indicating high lateral heat transfer efficiency and effective local overheating suppression. This study clarifies the fundamental cooling mechanism and quantitative contribution of IWF under natural circulation, providing mechanistic experimental evidence and benchmark data for the establishment, validation, and engineering application of IWF models in SFR natural circulation safety analysis.