Small-angle neutron scattering was used to observe the relaxation of the structure factor from a smaller to a larger fluctuation state after a temperature jump within the one-phase region of a miscible polymer blend. The blend studied is a 60/40 by weight mixture of a hydrogen bonded polymer blend which consists of hydroxy modified deuterated polystyrene and poly(butylmethacrylate). A peak clearly evolved during the transition from the initial state at Ti to the final state at T-f as predicted by linear as well as nonlinear theories. This relaxation process, which took more than 10 h, could be represented reasonably well by a simple linear approximation of the Cahn-Hilliard-Cook (CHC) theory without involving nonlinear calculations for the evolution of the dynamic structure factor. The measurement offers an alternative method for estimating the order parameter relaxation rate in the one-phase region. Also, this demonstrates experimentally the correct relaxation (to a higher fluctuation state) mechanism predicted by the linear theory of CHC type.