The nucleation rate of hydrogenated microcrystalline silicon (µc-Si:H) films deposited by plasma-enhanced (PE)CVD on hydrogenated amorphous silicon (a-Si:H) substrates is investigated through structural and electrical characterization, with special attention paid to the initial growth stage of µc-Si:H films. It is found that the nucleation rate of µc-Si is dependent on the thickness of the a-Si:H substrate. The µc-Si:H film exhibits a rapid nucleation on a thin a-Si:H layer, leaving a thin incubation layer at the µc-Si/substrate interface. This substrate-thickness dependence of the nucleation rate is proposed to be correlated with the stress inside the a-Si:H layer. The high interfacial stress existing in the thin a-Si:H layer facilitates the formation of high concentration, strained Si-Si bonds, which are responsible for the rapid µc-Si nucleation. The thick a-Si:H layer relaxes the interfacial stress through the formation of islands in the Stranski-Krastanow (S-K) growth mode, while the intrinsic stress is still low, resulting in a long nucleation process allowing for the intrinsic compressive stress to be accumulated that is necessary for the µc-Si deposited on it.