Owing to having a hollow tube structure, one-dimensional carbon nanotubes can be filled with various kinds of molecules.The encapsulated materials exhibit peculiar structures and properties because of spatial confinement effect of carbon nanotube walls.Confinement of materials in the nanometer scale spaces induces unique phenomena different from a bulk phase, such as chemical reactions, phase transitions. In this review, we introuduce the factors of chemical reaction inside carbon nanotubes,the reaction type and quasi-high pressure effect. We summarizes the carbon nanotubes can be used as a new type of nano-reactor on synthesis of special or precious materials. Finally, the research and application in this field are also discussed.