Recently several gravitational wave detections have shown evidence for compact object mergers. Three of gravitational waves are sourced by a black hole (BH)-BH mergers more massive than 30 solar mass and one is by neutron star (NS)-NS mergers. It is challenging to explain a high rate of such massive BH mergers, whose progenitors would be very metal-poor stars. Further, existing astrophysical models for neutron star mergers typically predict a lower merger rate than the observed rate. Hence the astrophysical origin of merging binaries is not well understood. One of the most plausible environments for massive BH mergers is galactic nuclei. Since galactic nucleus has a very dense stellar component, frequent two and three body interaction and resonant relaxation of the stellar component work for formation and evolution of binaries. Further, inactive phase, dense gas also plays a role in the formation and evolution. We found binary hardening in active galactic nuclei is faster than binary disruption when gas effects are considered. Then the merger rate in the galactic nucleus is increased significantly. We also have proposed a new channel for mergers of compact objects.