We investigated how the encounters between galaxies change their mass M and velocity dispersion σ. We carried out a series of direct N-body simulation of encounters of two spherical galaxies. In Figure 1, the relative change of energy ΔE/E are plotted against that of mass ΔM/M for various initial conditions. The filled and open symbols correspond to the cases of Plummer model and relaxed Hernquist model, respectively. Here β ≡ (ΔE/E)/(ΔM/M).