Micro- and nanoscale gas flows are analyzed theoretically and numerically. The analyses of gas flow similarity show that the gas flows at different scales can be similar only when the gas is treated as a prefect gas. If the gas density is so high that the density effect cannot be ignored, the three dimensionless parameters, Re, Ma, and Kn, which characterize the micro gas flow, are independent of each other and cannot be equal for flows at different scales, so the similarity breaks down. The critical density for the similarity failure can be analytically determined for each kind of gas. The analytical results were validated by numerical simulations. High density, high Knudsen number gas flows were modeled using a generalized Monte Carlo method based on the Enskog theory which considers both the density effect on the collision rate and the molecular repulsive and attractive interactions for a Lennard-Jones gas. The predicted transport coefficients agree better with experimental data than previous predictions. The simulation results show that when the gas density is higher than the critical density, the denseness effect alters the flow velocity and temperature fields from the direct simulation Monte Carlo results. Higher densities lead to greater deviation. (c) 2007 Elsevier Ltd. All rights reserved.