  We performed N-body simulations of a dust layer without a gas component and
examined the formation process of planetesimals. We found that the formation
process of planetesimals can be divided into three stages: the formation of
non-axisymmetric wake-like structures, the creation of aggregates, and the
collisional growth of the aggregates. Finally, a few large aggregates and many
small aggregates are formed. The mass of the largest aggregate is larger than
the mass predicted by the linear perturbation theory. We examined the
dependence of system parameters on the planetesimal formation. We found that
the mass of the largest aggregates increase as the size of the computational
domain increases. However the ratio of the aggregate mass to the total mass
$M_\mathrm{aggr}/M_\mathrm{total}$ is almost constant $0.8-0.9$. The mass of
the largest aggregate increases with the optical depth and the Hill radius of
particles.
