Energy dissipation is one of the most important properties of granular gas, which makes its behavior different from that of molecular gas. In this work we report our investigations on the freely-cooling evolution of granular gas under microgravity in a drop tower experiment, and also conduct the molecular dynamics (MD) simulation for comparison. While our experimental and simulation results support Haff’s law that the kinetic energy dissipates with time t as E(t)～(1+t/τ)?2, we modifyτby taking into account the friction dissipation during collisions, and study the effects of number density and particle size on the collision frequency. From the standard deviation of the measured velocity distributions we also verify the energy dissipation law, which is in agreement with Haff’s kinetic energy dissipation.