In order for Li-Si alloys to be used in Li-ion batteries as anodes, knowledge of their mechanical properties, such as Young's moduli, is crucial. Young's modulus of polycrystalline Li22Si5 was determined from nanoindentation testing. The value of Young's modulus was 35.4 +/- 4.3 GPa. This value is approximately one-half of the predicted value based on density functional theory calculations. This difference was not a result of the testing procedure or microstructural variables. (C) 2011 Elsevier B.V. All rights reserved.