A reaction tube for performing isothermal polymerase chain reaction therein is provided and includes an upper section, a lower capillary section, a linkage connecting the upper section and the lower capillary section, and a thermal conductor. The lower capillary section has an annealing portion, an annular heating groove, and a close end. The annealing portion is connected with the linkage, and the annular heating groove is connected with the annealing portion. The close end is connected with the annular heating groove. The thermal conductor tightly embraces the annular heating groove of the lower capillary section for conducting heat. Accordingly, a heat generated from the heating source will conduct to the annular heating groove of the lower capillary section of the reaction tube, a temperature gradient along the reaction tube is then provided in order to perform a thermal convection, thereby performing the isothermal polymerase chain reaction in the reaction tube.