The freezing behavior of a supercooled water droplet on a silicon surface treated with using various silanes was observed directly using a high-speed camera system. Freezing stability is best ensured by heterogeneous nucleation from a three-phase (solid-liquid-air)- contact line. Differential scanning calorimetry measurements revealed that the freezing temperature of a supercooled water droplet on a fluoro-alkylsilane monolayer was lower than that of alkylsilane. Interaction between the fluorocarbon and water molecule, in addition to structural characteristics of silanes, might play an important role in the heterogeneous nucleation of supercooled water. (C) 2007 Published by Elsevier B.V.