Micro-flow chemical processes, which are attracting great attention lately, offer more than the advantage of size. In microspaces, mass and heat transfer lengths are short, the specific interfacial area is large, and heat capacity is remarkably small compared with the situation in conventional scale reaction vessels. These characteristics make precise control of reaction conditions possible, and higher selectivity and conversion can be obtained. In some cases, reactions that are too dangerous to be carried out on a conventional scale can be safely done using micro-flow reaction systems. Recently, some researchers have developed basic model systems for solution-phase combinatorial synthesis by utilizing micro-flow chemical processes. There are two types of combinatorial synthesis systems using micro-flow chemical processes: parallel reaction systems and sequential reaction systems. In parallel reaction systems, components in starting libraries are simultaneously mixed in all possible combinations by using many micro reaction channels. In addition to needing as many pumps as the starting materials and as many reaction channels as members of the product library, complicated three-dimensional piping is necessary to connect the pumps and the micro reaction channels. In sequential reaction systems, segments of starting materials are sequentially injected and reacted in a single micro reaction channel in all possible combinations, The latter systems are usually smaller than the former, however, relatively large fluid control devices are necessary in both of them, and improvement of micro-fluid-control devices is a critical issue for putting micro-combinatorial-synthesis systems to practical use.