[en] The objective of the present study was to investigate experimentally and analytically the heat transfer of strongly heated gas flow in an annular passage with a major focus on the laminarization phenomenon. The following major conclusions were derived: (1) In the annular passage with only one heated wall, laminarization does not occur and heat transfer coefficient for strongly heated flow approaches a new asymptotic correlation, which is independent of the heat flux. (2) In the annular passage with both heated walls, laminarization occurs when the heat flux exceeds certain magnitude. (3) The flow pattern in the annular passage is classified into four regimes by Reynolds number and the heat flux. The present results proved that the fuel elements of the High Temperature Engineering Test Reactor (HTTR) of JAERI, does not exceed the allowable limiting temperature under the normal operating condition and abnormal and accidental conditions. (author)