Typically, O-rings are used to prevent penetration of dust and alien substances from entering a cylinder during motion. Moreover, O-rings are used to create an air tight seal around a stationary shaft. The stresses developed in O-rings depend on the squeeze rate, gap between the external diameter of the groove and internal diameter of the cylinder as well as internal pressure. In application, the stress distributions in O-rings can be very complicated and are almost always studied through experiment. Photoelastic experiment has been applied to the study of 3-dimensional stress distributions in O-rings. The loading device used in photoelastic experiment is important. Its function is to apply a uniform squeeze rate and internal pressure on the O-ring and to allow uniform squeeze rate to be controlled. In this research, a loading device was developed to perform these functions. The validity of this loading device was confirmed through the stress distribution, the configuration change and the contact length of the O-ring. When the squeeze rate was constant, the upper and lower contact lengths of the deformed O-ring were almost equal. When internal pressure was applied to the O-ring, while under a uniform squeeze rate the upper contact length increases slightly with increase in internal pressure, while the lower contact length of the O-ring is constant with an increase in internal pressure.