Synthesis experiments and numerical analyses were conducted for competitive-consecutive reactions in microchannels in order to verify an evaluation method of chemical reactions by dimensionless numhers. The bromination reactions of dimethylphenol were used as models for the competitive-consecutive reactions. It was experimentally demonstrated that the product yield of the primary reaction depended on two dimensionless numbers: the mixing-reaction number which is the ratio of the diffusion time to the reaction time in the chemical reactions and the ratio of the secondary reaction rate to the primary reaction rate. The product yield of the primary reaction increased with decreasing mixing-reaction number and decreasing reaction rate ratio. The reaction rate constants of primary and secondary reactions could be evaluated by comparing the experimental results with calculated results. The evaluation of the diffusion length necessary to achieve the target product yield became possible using the evaluation diagram of the product yield of the primary reaction.