In order to study the technique of blending gas-atomized steel powders, being liable to size-segregation, the determination of particle size distribution was carried out by using a radioactive tracer. A fraction of 10% cobalt high speed steel powder was activated by neutron in the Hitachi Training Reactor, thus yielding Co-60 to be traced. The activated powder as the tracer was admixed to the remainder by two different mixers, i.e. a ball mill and a static mixer; the composition of the mixtures could be analysed radiometrically.The static mixer proved to have better blending effect compared with the ball mill, irrespective of local segregation observed during handling of the static mixer.