Mixing in micro space is categorized into two concepts : rapid mixing and precise diffusion control. In this paper, the methodology for designing nano-particles was summarized based on these mixing concepts. First, the processing logic for nucleation by micro mixer was discussed in light of the typical experimental result. On the other hand, it was shown that a precise diffusion control by designing a micro space had a significant effect for individual strict control of nucleation and particle growth processes. The results shown here clearly indicate that micro reactor technology will be a powerful tool for nano-processing.