The roughness of monatomic A- and B-type step edges on 0.5° misoriented Si(001) has been analyzed on an atomic scale with scanning tunneling microscopy. On small length scales, measured along the step edge (<200 Å), one-dimensional random-walk behavior is observed for both types of step edges. For the rough B-type step edge we also found evidence for waviness of the edge. The period of this wave is about 100 dimer-row spacings (≊750–800 Å). The energetic step-step interaction and entropic repulsion, which both scale as 1/L2 (L is the average terrace length), are estimated to be about 0.03–0.06 and 0.2 meV per dimer-row spacing, respectively, for a 0.5° misoriented surface. These interactions are approximately three orders of magnitude smaller compared to the kink formation energies which are 0.1–0.2 eV. Despite the weak strength of energetic and entropic step-step interactions, these long-range interactions have a profound effect on the step-edge morphology, e.g., the distribution of terrace lengths and the long-range waving of the rough B-step edge.