We have measured the fluorescence lifetime of Nile Red (NR) in various polymers and found that the fluorescence lifetimes of NR depend strongly on Young's modulus rather than polarity of media. The results indicate that the twisting motion of the diethylamino group in NR occurs in the excited state in the free volume of polymers. Our results show that the fluorescence lifetime of NR can be used to estimate Young's modulus or the free volume of a polymer. (c) 2009 Published by Elsevier B.V.