It is well known that the supramolecular assembly of block copolymers with associated low-molecular-weight additives leads to rich phase behaviors in both bulk and thin films. In this study, we demonstrate that the supramolecular assembly is also an efficient approach to control the microdomain orientation in block copolymer thin films. We investigated the effect of the content of low-molecular-weight additives on the microdomain orientation in supramolecular thin films formed by poly(styrene)-b-poly(4-vinylpyridine) (PS-b-P4VP) with hydrogen-bonded 3-pentadecylphenol (PDP) and found that at low grafting density of PDP onto P4VP chains, the PS microdomains are parallel to the surface, while above a critical grafting density, the orientation is switched to be perpendicular. Such orientation transitions are observed for both cylindrical and lamellar structures, as confirmed by AFM, TEM and GISAXS. We propose that the microdomain orientation is governed by the combining effect of interfacial energy and the spatial distribution of additive molecules that varies with grafting density.