Barite (BaSO4) is one of the common scales in oil and gas production. Extensive work has been conducted to study barite nucleation and inhibition at temperatures below 100 °C. However, with the advance in deepwater exploration and production which can encounter high temperature conditions, a better understanding of barite scaling risk at high temperature (e.g., >150 °C) becomes essential. In this paper, a systematic study was conducted to explore barite nucleation kinetics from 70-200 °C in synthetic brines containing phosphonate (0-10 ppm) or polymeric (5-10 ppm) scale inhibitors. A 2-hour protection time with or without any detectable barite nucleation was employed to define the scaling risk. To detect barite nucleation, two novel apparatus were developed, a modified dynamic flow loop and a batch reactor. The modified dynamic flow loop has a retention time of up to 4 hours and is ideal to carry out experiments at above 100 °C. Ba concentrations in the effluents were monitored to determine barite nucleation more precisely compared to traditional “tube blocking” technique. The new batch reactor employs our newly developed laser detection method, a transparent pressure tube, and an oil bath. The transparent pressure tube allows laser to pass through and can withstand 150 psi pressure at 150 °C, which therefore provides an efficient and convenient approach to study the precipitation kinetics of scales and evaluate inhibition efficiency of inhibitors at high temperature. Constant inhibitor concentration isoplethes of diethylenetriamine pentamethylene phosphonic acid (DTPMP) for barite inhibition were constructed based on our experimental data. The results of this study can facilitate the selection of an appropriate DTPMP concentration for scale treatment for high-temperature oil and gas production.