The size of mosaic blocks in lead powder was determined on the basis of the primary extinction effect of x rays. Integrated intensities of nine reflection lines in the Debye‐Scherrer spectrum with CuK&agr;radiation were measured with a Geiger counter spectrometer. The mosaic size was calculated by comparing these values with theoretical values of the intensities. Correction for absorption of x rays in the specimen powder was especially important because of the large absorption coefficient of lead. The order of magnitude of the mosaic size was found to be 10−4cm. From this value, the dislocation density was calculated to be of the order of 108cm−2. The dislocation density decreased slightly when the specimen was annealedin vacuoat 150°C for a long time.