The diffusion-based micro flow of a T-sensor with three inlets in which various species are injected was simulated numerically. The results show that the Reynolds number is an important factor of affecting the efficiency of diffusion The smaller the channel width is, the more strongly the species diffuse. The velocity gradient across the channel width plays a key role in the diffusion of species. The conclusions are helpful to the design of micro-fluidic devices and the analysis of data collected from such devices.