Recently, high-pressure resin transfer molding (HP-RTM) process which can produce high quality products in a short period of time was developed especially for automobile industry and some of the products has been started to be mass-produced. Accordingly, there are various studies to develop and optimize the HP-RTM process with the high speed resin injection. On the other hand, the fiber preform can deform when the resin is injected by high pressure in the HP-RTM process, which causes defects and poor appearances in the product. In this study, measurement system of preform deformation and mechanism for various deformation modes is investigated. There are three kinds of important forces, which are friction between fiber and mold, fluid pressure by flow and in-mold stiffness of fiber preform. The forces are measured by newly designed measurement equipments. Two kinds of deformation modes of fiber preform are observed. Slip is occurred in a whole fiber preform when the fluid pressure is greater than the friction, and wrinkle is occurred in a front part fiber preform when the fluid pressure is greater than the in-mold stiffness. Various deformation modes such as slip, wrinkle are observed and the corresponding causing mechanism is proposed by comparing the measured values of three important forces. key word : HP-RTM, composite, fiber deformation, friction