A computer simulation is employed to demonstrate, in a two‐dimensional growth model, that a vapor‐deposited thin film of low adatom mobility undergoes a sudden change from a porous columnar microstructure to a densely packed film if the substrate temperature is increased to a certain value. The temperature where this structural transition occurs is shown to be related to the lower boundary temperature of the empirical structure‐zone model. The dependence of the transition temperature and range on the vapor deposition rate is discussed.