The self-complementary hydrogen bonding ureido-pyrimidinone (UPy) motif is widely used in the design of supramolecular polymers because of its high dimerization constant. Lateral aggregation into fibrous structures is achieved by the addition of urea functions close to the UPy end group of low-T(g) oligomers, yielding supramolecular thermoplastic elastomers. The rate of fiber formation is critically dependent on the substituent at the five- and six-positions of the UPy unit. Here the aggregation behavior in the solid state is disclosed for a series of molecules with the commonly used methyl, the optically pure (S)2,7-dimethylheptyl and (S)-1-methylpropyl, and the racemic 1-ethylpentyl group at the six-position. The rate of nanofiber crystallization from the melt was investigated with a variety of techniques, including SAXS, WAXS, AFM, DSC, IR, and CD spectroscopy. As a result, the different stages involved in the nanofiber formation were elucidated. The nanofiber formation is a hierarchical process starting from the phase-separated melt with the dimerization of the UPy-units. For the lateral aggregation into high aspect nanofibers, both a nonsubstituted five position and urea functionalities are required. The nanofiber formation is the result of ID stack formation accompanied by secondary nucleation of multiple stacks. The stack-to-stack distance within a nanofiber is dependent on the size of the UPy-substituent, which demonstrates that the substituents are in-between the stacks in the nanofibers. The results also demonstrate that stack and nanofiber formation is slowed down and suppressed by a branching of the six-substituent close to the UPy motif, whereas the presence of stereochemical isomers further suppresses this aggregation from the melt. These detailed insights into the kinetic behavior of nanofiber formation pave the way to create adaptable supramolecular materials.