The penetration of a water drop into hydrophobic pores reflects its instability on a porous surface. To understand the mechanism of penetration and to predict the behavior of such a drop, an investigation was conducted through experimental study combined theoretical analysis. Water drops with volumes from 0.5 to 15 L were examined on Polydimethylsiloxane (PDMS) substrates containing pores of 800 vim and less in diameter. Results showed a critical condition at which a drop starts to penetrate into a certain sized pore. The critical condition presents a parabolic relationship between the volume of a water drop and the size of a hydrophobic pore. This behavior was due to a net force resulting from Laplace pressure, and capillary pressure. This force was found to be affected by the porosity, wetting angle, and there after the critical condition. The finding of this research will be beneficial for future design of structured surfaces. (C) 2016 Elsevier Inc. All rights reserved.