Abstract: Using a newly developed velocity map imaging apparatus coupled with a magneto-optical trap, we measured ion temperature and Coulomb coupling parameter of ultracold neutral plasma (UNP). We carried out numerical simulation in early evolution of UNP. The experimental and simulation results showed that the disorder-induced heating mechanism led to a rapid rise in ion temperature during the ion equilibration, and the Coulomb coupling parameter dropped to 2.5. Furthermore, we found that the plasma expansion velocity would quickly approach the thermal velocity of ions in the ion equilibration, which results in a higher Coulomb coupling parameter than experimental values.