The tensile strength of numerous whiskers of Fe, Fe-Cu, Fe-Co, Cu and Cu-Co with diameters ranging from 20 to 300 microns were measured at room temperature and at an elevated temperature. The results show that the strength of the dispersion type of binary metal whiskers such as Fe-Cu and Cu-Co whiskers is higher than that of single-metal whiskers, while the strength of the solution type such as Fe-Co whiskers is not so. On the basis of measurements on stress-strain curves and metallographical observations, it is suggested that the high strength of dispersion type Fe-Cu whiskers may be due to the appearance of a high strength dispersively mixed layer of iron and copper, and the improvement of plasticity may be due to the appearance of an axle whisker of copper having a better plasticity.A discussion was made on the basic processes involved in the thickenning of binary metal whiskers of either type. It is suggested that the procedure adopted in growing dispersion type of binary metal whiskers may serve as a useful guide in preparing high strength composite materials with improved plasticity.