Abstract: Understanding the features of micro-scale gas flows is essential to designing and optimizing micro-electro-mechanical systems(MEMS). Research on micro-scale gas flows originated from the famous experiments carried out by Knudsen and Millikan, respectively, i.e. the measurements of mass flux through a plane channel and drag coefficient of a small sphere, at the beginning of the 20th century. The experimental results revealed the significant effects of scale influence (rarefaction) on the gas flow characteristics. In the slip and transition regimes, the similar parameters of gas flows are the Knudsen number and Mach number, and therefore, there is a possibility to utilize the principle of similarity, i.e. increasing the geometric scale while decreasing the pressure, to solve the difficulty associated with the measurement in MEMS experiments. An information preservation(IP) technique was proposed by us to address the serious statistical scatter encountered in Monte Carlo direct simulation of low-speed gas flows. The IP technique may reduce the statistical scatter efficiently, and has been successfully applied to many micro-scale gas flows.