An application of modal control theory, recently developed by the authors, to the control of a double-pipe heat exchanger is presented. Since the flow rate of cold fluid is selected as a manipulated variable, the process model is described by a set of nonlinear partial differential equations. After the lineal approximation of the original model near an operating point, the finite integral transform is applied to derive a slate space representation. A controller is designed based on the derived reduced state space model. The result is demonstrated through the experiment with a small-sized double-pipe heat exchanger.