Defect centers in diamond has outstanding properties, including long coherence times and the presence of a nuclear spin bath, which make them highly promising for quantum applications. However, the technology for large-scale integration of defect centers is still in its early stages. There are various methods for the integration of diamond centers for quantum applications. Here we focus on the hybrid integration of diamond nanophotonic structures in three aspects: 1) adiabatic coupler design, 2) pick-and-place integration of diamond chiplets with silicon nitride (SiN) photonic platform, and 3) preparation of a diamond membrane for nanofabrication. An adiabatic coupler has been designed for an existing diamond chiplet. The optimal performance has been obtained at 12 μm taper length and 120 nm tip width with an insertion loss of 0.16dB, and coupling efficiency of 78.2%. It has also been demonstrated that much better performance could be obtained by reducing the diamond waveguide thickness from 485 nm to 200 nm, giving an insertion loss of 0.094dB and a coupling efficiency of 97.2%. The optimal taper lengths in this case have been determined as 12 μm for the SiN waveguide and 8 μm for the diamond waveguide. The effect of surface treatment on the pick-and-place transfer has been investigated as well. The time dependence of the contact angle of various surface treatments applied on the SiN surface has been characterized. It has been found that the optimal pick-and-place condition can be obtained with HMDS treatment at a contact angle range of 20∘ −30∘ measured by water, corresponding to a time between 1 and 2 hours after the treatment. One issue found in pick-and-place experiments is the roughness of the diamond nanophotonic chiplet bottom surface. Etching of diamond on insulator (DOI) substrates has been performed for two kinds of diamond samples. It is found that the minimum surface roughness achievable by Ar/Cl2 + Ar/O2 etching is limited by the initial surface quality. The average surface roughness was lowered from 5.71 nm to 1.19 nm using a higher-quality diamond.