Abstract The dynamics and mechanism of columnar grain growth of pure iron with small equiaxed grains were experimentally investigated under directional annealing. The results prove the existence of both a lower- and an upper limit of the withdrawing velocity for columnar grain growth. Importantly, there is an optimum withdrawing velocity at which the largest aspect ratio of columnar grains can be obtained. A kinetic approach is suggested to describe the growth of the columnar grains. The aspect ratio of the columnar grains can be successfully predicted by this approach. The mechanism of the columnar grain growth essentially can be described as selective growth or competitive migration of the grain boundaries. As a result of selective growth, the boundaries between the columnar grains abound in low-energy boundaries. Small island-like grains inside the columnars grain can be left over due to the low mobility of their small-angle boundaries or twin boundaries.