Flow microreactors can provide a revolutionary change on organic synthesis, because of the following characteristic features derived from their small size and flow nature: 1) fast mixing, 2) temperature control, 3) highly strict control of reaction condition. By taking advantage of these feature, chemical reactions which are very difficult or impossible in flasks can be achieved in flow microreactors. We have applied this protocol to selective monobromination of 2,2'-bithiophene with N-bromosuccinimide. After various trials of controlling reaction conditions, the yield of the undesired di-brominated product (5,5'-dibromo-2,2'-bithiophene) could be reduced to 1−2%. The most important factor in the microflow system was the combination of the flow rate and the equivalent of N-bromosuccinimide. This article is described the details of the progress to achieve good selectivity.