DC arcjet plasma was diagnosed by spatial resolution optical emission spectra (OES). A flexible sample holder was thus constructed to avoid the fluctuation of gas phase species. By this flexible sample holder, single crystalline diamonds were grown on natural diamond substrates without the poly-crystallization. The influence of growth parameters, the total feed gases flow and the substrate movement velocity, on the morphology of the growth surface was studied by the scan electron microscope (SEM). Growth steps and hillocks were observed on as-grown surfaces. There existed compressive stresses in growth surfaces detected by Raman spectra, from which the occurrence of the hillock was thought to be ascribed to the development of Asaro–Tiller–Grinfeld (ATG) type morphology instability.