Silicon is one of the most promising anode materials for lithium ion batteries because of its extremely high theoretical capacity. However, silicon suffers from mechanical degradation caused by huge volume change and unstable solid electrolyte interphase (SEI) layers. Herein, we report an in situ electrochemical atomic force microscopy (EC-AFM) method to directly visualize the surface topography and analyze Young's modulus of micron-sized (Micron-Si) and nano-sized (Nano-Si) silicon electrodes. Our results show Micron-Si electrodes experienced volume expansion and contraction process which resulted in continuous growth of a thick but soft SEI layer on the surface. In contrast, Nano-Si electrodes demonstrate a thin SEI layer due to absence of volume expansion and contraction process. Young's modulus value shows that the SEI film of Nano-Si electrodes is harder than that of the Micron-Si electrodes. Ex situ XPS analysis reveals that Nano-Si electrodes are composed of mainly inorganic components particularly LiF and carbonate-like species which might contribute to the increased hardness. Results from present study may be helpful to build better SEI layer with good Young's modulus to buffer volume expansion of Si. (C) 2018 Published by Elsevier B.V.