In order to propose a new simple method for the fabrication of a microchannel covered with a graphene layer on a diamond substrate, the application of a phase transition via a solid solution phenomenon, that is, the dissolution of a thermodynamic-metastable phase and the deposition of a thermodynamic-stable phase, was performed in this study. A CVD diamond film in contact with a Ni wire (100 micrometers in diameter) was heated under flowing H2. The experimental results in this study proved that a micro-channel (ca. 5 micrometers deep) covered with a graphene layer, the stacking number of which was more than 6, can be successfully patterned on a diamond substrate. The etching rate was estimated to be ca. 1.3-1.4 μm h-1. It was proved based on the experimental data from this study that this method can be used to fabricate a microreactor on a diamond substrate although the technique may be refined.