Stress concentration due to the circular hole within the mechanical part is concerned during design phase. The stress concentration of the circular hole within the mechanical mart can easily derived by the photo-elastic technique if the testing specimen is sufficiently developed. The testing specimen using the characteristic of the photo-elastic coefficient, with actual form like the mechanical part by rapid prototyping forming is thus used in this paper. The rapid mold for prototype is made by the RTV-2 silicone gel associated with the vacuum chamber to remove the air within it. Totally twelve specimens divided into two sets, the first set with the machined holes by milling at the centroid of the specimens with diameters of 5, 8, 10, and 12 mm. The second set with the formed holes by molding with the previous diameters. The testing scheme is by way of the simply supported beam where the load is imposed on the middle of the specimen. Results show that the smaller diameter of hole on specimen provides a larger stress concentration effect. When the diameter of the hole is small, the stress concentration effects caused by the machined hole and the forming hole have almost no difference. However, the stress concentration effects of the larger diameter of the machined hole and of the formed hole give significant difference, the stress concentration caused by the machined hole is much greater than one of the formed hole. In this paper, the actual shape of the specimen which is as same as the original mechanical components can be quickly obtained by the rapid prototyping technology for the photoelastic technique to reveal the experimental mechanics of real applications.