The pure cubic GaN(c-GaN) has been grown on (001)GaAs substrates by ECR-PAMOCVD technique at low temperature using TMGa and high pure N2 as Ga and N sources, respectively. The effects of substrate pretreatment conditions on quality of cubic GaN epilayer are investigated by the measurements of TEM and XRD.It is found that hydrogen plasma cleaning, nitridation and buffer layer growth are very important for quality of cubic GaN epilayer.