Hillocks on c-plane homoepitaxial GaN epilayers were investigated. They were observed on epilayers grown on [1 (1) over bar 00] direction miscut free-standing GaN substrates with miscut angle not larger than 0.2 degrees and were absent when substrate miscut angle increased to 0.4 degrees. Atomic force microscopy (AFM) and cathodoluminescence measurements reveal a close correlation between hillocks and dislocation clusters, while hillocks are absent on layers grown on GaN substrate free of dislocation clusters. We believe that the hillocks originate from spiral growth around dislocation clusters. Larger strain induced by dislocation accumulation may be responsible for the hillock formation around dislocation clusters. (C) 2013 Elsevier B.V. All rights reserved.