Crystallization of vapor-deposited thin films of n-butane and n-pentane has been investigated using temperature-programmed time-of-flight secondary ion mass spectrometry. The morphology of thin n-butane (n-pentane) films changes at around the calorimetric crystallization temperature of 65 K (85 K) as a result of crystallization of the supercooled liquid. The morphology of the crystal grains of n-butane changes at 85 K; the butane molecules permeate through porous amorphous-solid-water films above this temperature. The crystal grains of n-pentane are smaller in size than those of n-butane, forming a smoother crystalline film. However, the crystalline n-pentane film dewets abruptly at higher temperatures, depending on the film thickness. The liquidlike nature of crystalline n-pentane (n-butane) is attributable to premelting (coexisting second liquid).