The mechanism and process of the thermally induced order-order transition (OOT) of a polystyrene-block-polyisoprene diblock copolymer from cylinders on a hexagonal symmetry (hex-cyl) to spheres on a body-centered cubic symmetry (bcc-sphere) were investigated by means of time-resolved observation of high-resolution small-angle X-ray scattering (hr-SAXS) profiles obtained with a Bonse-Hart type USAXS apparatus. The USAXS apparatus enabled us to precisely study changes in the scattering peak profiles with time induced by the OOT between the two equilibrium morphologies of hex-cyl and bcc-sphere with a high resolution with respect to magnitude of scattering vector q. The peaks due to hex-cyl and bcc-sphere, which are closely located and hence overlapped one another in q-space when they are observed with conventional SAXS apparatuses, are well resolved into separated peaks with the hr-SAXS, which gave a new insight into the OOT process and mechanism as elucidated in the text.