In order to elucidate the aggregation mechanism of triphenyldiamine (TPD) molecules used as a hole transfer layer in organic electroluminescent devices, morphological observation of thin films formed on KBr, indium tin oxide and quartz (fused silica glass) has been carried out. The molecule was evaporated from a Knudsen-type cell and deposited onto these substrates in vacuo, Both temperatures of furnace (T-f) and substrate (T-s) were precisely controlled in the ranges of 200-250 degrees C and 50-124 degrees C, respectively. Thin films had no characteristic X-ray diffraction peak, characteristic of an amorphous film. The small aggregates formed on the substrates were observed by using high-resolution scanning electron microscopy after being covered by a conducting uniform film of OsO4. The molecular aggregates of TPD are hemispherical and their size and density depended on the deposition conditions such as the kinds of substrate, substrate temperature and the furnace temperature. The dependence of diameter and distance between adjacent aggregates on the substrate temperature reveals the growth mechanism and surface migration of TPD.