The selectivity of temperature-sensitive exothermal reaction, especially for a fast reaction system, is hard to control in chemical synthesis process., Traditionally, reducing the reaction temperature is applied to control byproduct. Nowadays, the confinement of the reaction temperature can be released by using microreactors. To demonstrate this, the fast exothermal reaction between cyclohexanecarboxylic acid and oleum was chosen as an experimental system in this work. An adiabatic micropore dispersion capillary reactor was developed to control the reaction selectivity at relatively high temperature. The results showed that 5-30 mu m dispersed scale of oleum was reached in this microreactor, and nearly 100% selectivity was obtained at a relatively high product temperature, 50-60 degrees C, with 100% conversion of oleum.