Atomic force microscopy was employed to study the microtopography on the {001} surface of potassium perchlorate single crystals obtained by the gel method. This surface plays an important role on the growth of potassium perchlorate single crystals. The results show that the potassium perchlorate single crystals display various growth mechanisms at different supersaturations, including the twodimensional nucleation growth mechanism, threedimensional nucleation growth mechanism, layernucleation growth mechanism and stack growth mechanism. The effects of supersaturations on the growth mechanisms could be understood from a thermodynamic viewpoint. The shape and orientation of the twodimensional nuclei and the orientation of growth steps were observed. The results demonstrate that the impurities in the growth system display negative effects on the single crystal growth.