In the framework of the cryogenic cooling system design of a large superconducting magnet under construction at CERN-Geneva, heat transfer in two-phase He I natural circulation loop has been investigated experimentally. The experiments were conducted on a 2 m thermosiphon loop with copper tube of 10 mm inner diameter uniformly heated over a length of 0.95 m. All data were obtained near atmospheric pressure. Evolution of the exit vapour quality and wall superheat as a function of heat flux are presented and analyzed. A comparison between the two-phase heat transfer coefficient h(TP) determined in our study and the most relevant correlations available in literature is made. Further, we predict h(TP) With a correlation based on the combining effects of forced convection and nucleate boiling by a power-type asymptotic model. Finally, we present the boiling crisis study and we propose a critical heat flux correlation as a function of channel height to diameter ratio (z/D) to model our experimental results. (c) 2007 Elsevier Ltd. All rights reserved.