A novel viscous micropump consisting of a cylindrical rotor eccentrically placed inside a microchannel is simulated by the two Volume-CAD (V-CAD) framework-based flow solvers, i.e., the direct simulation Monte Carlo (DSMC) package (named as V-DSMC) and the Navier-Stoke solver (named as V-Flow). V-DSMC is used in the case of the pump applied to gas, while V-Flow is applied to model the pump in the case of liquid working medium. The pumping performance curves under different liquid media with the variation of Reynolds number, as well as under different eccentricity factors are obtained. The performance and the flow filed characteristics are very sensitive to the tangential momentum accommodation coefficient in the case of gas medium. Three recirculations exist in the flow field, and the sizes of recirculation are different at the different operating points in a performance curve.A significant challenge that must be overcome to enable scientific and industrial applications of MEMS and NEMS (micro/nano-electro-mechanical systems) and LOC (lab-on-a-chip) technologies, the latter being an idea for miniaturized instrumentation and control systems, is the transport and pumping of small quantities of fluids. At the macroscale continuous mechanical pumps (i.e., excluding positive-displacement pumps) are based on conventional centrifugal impellers, but these conventional methods do not work at small scales for which the Reynolds number is very small, even less than unit, since centrifugal forces are negligible at this range. However, at small length scales a novel pump known as the viscous pump has been developed by Sen et al [1] . It is suited for hauling various fluids in microducts from gas to liquid. The simplicity in the design of the viscous pump is its major attractive feature for MEMS, NEMS and LOC applications. The viscous pump consists of a transverse-axial rotor eccentrically placed in a channel, so that the viscous resistance between the small and large gaps as well as between the cylinder and the channel walls generates a net flow along the channel. Several numerical investigations appear in Refs. [2][3][4][5][6][7][8][9]. These studies in general model the viscous micropump using continuum assumption-based Navier-Stokes equations. In the case of liquid, the molecular length scale is small, and the continuum model may be still valid [10] . However, for a gas at the micro or nano scale, the rarefaction effect, the surface dominant effect and the compressibility effect may become significant, thus the flow solver must be able to accurately reflect the unique properties at this scale and the Navier-Stokes equations may not be available any more.The Volume-CAD (V-CAD) system as feature of Kitta Cube [11] can act as the platform for storing and handling both of the geometry data and volume data by representing them as attributes of a set of cells. The generated Cartesian background grids offer an efficient and elegant way of modeling complicated surface that cannot be described analytically. Through the V-CAD system...