Abstract: In some processes of resin matrix composites, such as a hot pressing process and RTM, the resin flow is an important phenomenon which can be detected by resin pressure. Monitoring resin pressure can obtain a lot of significant information for the optimization of the processing parameters and the control of the quality of composite parts. In this paper, a resin pressure measuring system for hot pressing processes was established based on the principle of pressure transfer in liquid. The variation of resin pressure in the process of unidirectional glass fiber laminates was investigated. By using the measuring system, the results show that the accuracy and the dynamic respondence of the system can meet the requirements of resin pressure testing in hot pressing processes, which is valuable for the study of the resin flow behavior in the processing.