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For the vast wooden house built-up areas in Japanese cities, Firebreak Belts (F.B.B.) are thought practical and effective against a post-earthquake urban fire. When a wooden house area is guarded by a vertical type of F.B.B. such as elevated traffic ways, a row of fireproof buildings, etc., this area might be exposed to the radiation mainly from the upper part of flame. The existing calculation method which assumes the homogeneous distribution within an imaginary plane of heat source should be reconsidered and the difference between upper and lower parts of flame in raqiation intensity must be discussed theoretically and empirically. In this paper , after the descriptions on the background and purpose of study, the outline of a full scale fire experiment are explained and the data of its external radiation are analyzed. Finally, a calculation model of the radiation from a big urban fire is proposed for the evaluation of effect of F.B.B.