Abstract: Few-body dynamics is an important process that controls the evolution of dense stellar systems such as young star clusters, massive globular clusters (GCs), ultra-compact dwarfs (UCDs) and nuclear star clusters (NSCs). It is also a key process to produce exotic objects including blue stragglers, X-ray binaries and Gravitational wave progenitors. To understand how star clusters and exotic objects form and evolve, the star-by-star N-body simulations are useful. But it is very time-consuming to simulate massive systems. To overcome this bottleneck, we developed the PeTar code, which is a high-performance N-body code that can be 10 times faster than the previous fastest code (NBODY6++GPU). The significant advance of performance opens a new window in the research of stellar dynamics. By using it, we find that there is the crucial issue hidden in the fast Monte-Carlo method which was frequently used to study the statistical properties of GCs and the dynamical origins of gravitational wave events. Since the code can handle much larger number of stars, we can study the formation of IMBH/SMBH via hierarchical mergers of BHs in NSCs. By combining the hydrodynamic code, we can study how few-body dynamics and binaries affect the star formation. This can be an important mechanism to form multiple stellar populations in young star clusters and old GCs.