Experiments were made on the growth of copper and iron whiskers by means of vapour reduction. The mechanism of growth was studied in order to find out the effective procedures of growing thick and long whiskers with very high strength. The growth of whiskers was found to be facilitated when the crystal structure of the grow-boat material is similar to that of the whisker. This led to the conclusion that the growth of whiskers is through a mechanism involving an axial screw dislocation. On the basis of observations on the mode of distribution and the direction of growth of the copper and iron whiskers grown on the wall of the boat, it was concluded that the growth of these whiskers proceeded from the tip. Furthermore, experiments showed that the cuprous (or ferrous) chloride vapour was preferentially reduced by hydrogen at the tip of the whisker, presumably because of the catalyzing action of the surface step produced at the tip by a screw dislocation.Observations on the growth of thick copper whiskers with a diameter of 50-120 microns revealed laminar growth, growth in piling up forms and in layers. Such peculiar growth behavior may possibly be associated with an alternation of one-dimensional and two-dimensional nucleation and growth. The possibility of growing a piece of bulk crystal essentially free from dislocations by suitably selecting and controlling the condition and mode of growth was discussed.