The understanding of volume change mechanism of silicon electrode is necessary to design a new negative electrode using silicon-based active materials. Here, the drastic volume expansion of one silicon secondary particle with gm-size was in-situ observed in order to find apparent volume expansion ratio during the first charging by using single particle measurement technique. The apparent volume expansion accompanied with the first lithiation is much larger than theoretical expectation due to the agglutination state and anisotropic property. The importance of direct observation with the single particle measurement has been affirmed for understanding the characteristics of silicon electrodes. (C) 2013 Elsevier B.V. All rights reserved.