We demonstrated, using grazing incidence wide-angle X-ray diffraction (GI-WAXD), sum frequency generational spectroscopy (SFG), and atomic force microscopy (AFM), that the aggregation state and molecular ordering of polymer brushes with crystalline side groups (i.e., poly(n-octadecyl methacrylate)) can be well controlled by their grafting densities (sigma(p)). The long-range ordered structure, which is the lamellar structure formed by rodlike n-octadecyl (C18) side groups, sequentially changes with increasing grafting density from the standard lamellae with a periodicity (d(lam)) of 3.0 nm at low sigma(p), to shrinking lamellae with d(lam) = 2.6 nm, and then to a structure lacking lamellar ordering. This regulation is ascribed to the restricted conformation and suppressed lateral mobility of the densely grafted chains and most importantly to a particular spatial confinement imposed by the limited end-to-end distance of the tethered chains, which hinders the long-range ordered organization of the bulky side groups. Meanwhile, we also showed that such a confinement effect occurring in densely tethering chains can be mitigated by introducing a soft grafting substrate.