Nanotechnology attracts much attention due to not only its useful engineering applications, but also their superior material performances. As the dimensions of the devices become smaller and smaller, however, understanding the mechanical properties of materials at sub-micron length scales becomes more challenging. This is driven by the knowledge that many mechanical properties at the sub-micron scale differ from those at the macroscopic scale and, in addition, mechanical deformations are coupled with other concurrent physical phenomena, such as diffusion, chemical reaction, and phase transformation. For the reliable design of MEMS or NEMS devices, nanotechnology has always called for an understanding of the mechanical behaviors of materials at small length scales. In this talk, size dependent plasticity in metallic micropillars will be discussed in terms of dislocation mechanics. Considering dislocation sources from the surface, micropillar plasticity will be analyzed using three dimensional dislocation dynamics model to understand how dislocation behavior relates to mechanical response in metals. In addition, We also have performed dislocation dynamics simulation to investigate the size effect in bi-crystalline FCC micropillars, taking account for the role of the grain boundary as both a source and a sink. Our simulation results show the smaller size effect in bi-crystalline micropillars, showing hardening for larger samples, softening in smaller samples due to the existence of grain boundary.