The international organization for standardization (ISO) has defined bubbles with the diameter between 1~100 μm as microbubbles in 2017. According to the above definition and the fact that microbubbles are easily affected by surface active materials, the mass transfer rate of single free rising microbubbles with various diameters under the influence of surface active materials was studied by direct numerical simulation. It was found that the velocity and mass transfer rate of the microbubbles were in good agreement with the theoretical results of creeping flow recommended by Clift et al. The adsorption of surface active materials on the surface of microbubble reduced the liquid-side mass transfer coefficient. However, for microbubbles that were greatly affected by surface active materials, the liquid-side mass transfer coefficient increased with the decrease of bubble size, which was different from the trend of clean microbubbles that decreased first and then increased. Therefore, in the application where the effect of surface active materials cannot be excluded, further reduction of the initial size of microbubbles can not only increase the specific surface area of the gas phase but also further increase the liquid-side mass transfer coefficient of the bubble, and the mass transfer capacity can be further enhanced.