Pulverization is a major cause of the capacity fade and poor cyclability of Sn-based anodes in lithium-ion batteries. We study the structural evolution of Sn nanoparticles during electrochemical lithiation-delithiation cycling by in situ transmission electron microscopy (TEM). The beta-Sn nanoparticles in the size range of 79-526 nm are lithiated to the crystal Li22Sn5 phase via a two-step mechanism, and no cracking or fracture, is observed, distinct from the lithiation-induced fracture in micron-sized Sn particles. Lithiation can induce the aggregation of small Sn nanoparticles with diameter in tens of nanometers, while delithiation can lead to the pulverization of large Sn nanoparticles. Similarly, the aggregation and pulverization of Sit nanoparticles are also observed daring sodiation and desodiation, respectively. The delithiation/desodiation induced pulverization of Sn nanoparticles is attributed to the high dealloying rate that results in cracks and voids in dealloyed Sn anodes. Based on our in situ TEM results, a size-dependent ' pulverization mechanism of Sn particles is proposed. Our work advances the mechanistic understanding of the pulverization and degradation mechanisms of Sn particle-based electrodes in Li-ion batteries. (C) 2014 The Electrochemical Society. All rights reserved.