This paper aims to investigate quantitatively the influence of gas-phase unsteadiness on the droplet vaporization process in sub- and super-critical environments. Two comprehensive models of high-pressure droplet vaporization, including a transient model and another assuming gas-phase quasi-steadiness, are presented. Both models are first compared with experimental data and then used to calculate vaporization processes of single droplets of different initial sizes for environmental conditions in which the ambient pressure and temperature range from 1-150 atm and 500-2000 K, respectively. The unsteady effects are quantified by introducing characteristic time scale ratios. It is shown that strong gas-phase unsteadiness exists during the early period of the vaporization process. The unsteadiness attains a maximum value in the gas near the droplet surface and decreases quickly to a nearly steady value within a short distance from the surface. With increasing ambient pressure, the unsteadiness increases nearly linearly at low ambient temperatures and rapidly at high ambient temperature. Gas-phase unsteadiness also increases with increasing ambient temperature and is affected even more strongly by temperature. Compared to the transient model, the quasi-steady model predicts a smaller regression rate initially and a larger regression rate during the later period. The differences between the predicted regression rates, and thus between the predicted vaporization processes, are magnified with increasing ambient temperatures and/or pressures. The vaporization process predicted using the quasi-steady model reaches the critical mixing state earlier than that predicted using the transient model. These conclusions also apply for the vaporization processes of single droplets of different initial sizes.