The collision of a liquid droplet with a hemispherical droplet on a horizontal smooth solid surface is studied experimentally as well as numerically. The hemispherical droplet is in the static state on the surface at room temperature and its volume is the same as an incoming droplet. The incoming droplet impinges vertically on the top of the hemispherical droplet. Qualitative and quantitative observations of the collision behavior of the liquid droplets are performed by means of a flash photographic method. The effect of impact velocity of the incoming droplet is investigated. It is found that a circular thin liquid film like a crown is formed around the bottom of the incoming droplet after the collision. The magnitude of the circular thin film becomes large with increasing the impact velocity. The collision behavior of liquid droplets is also analyzed numerically. The Navier-Stokes equations for incompressible viscous fluid in the axisymmetric coordinate system are solved by a finite difference method. The effects of viscosity, surface tension, and gravity are taken into account. The numerical results agree reasonably well with the experimental data. The hydrodynamics of the liquid is investigated to understand the physics of phenomena.