The core diameter of a single-mode fiber is about 8 to 10µm. Any slight misalignment or deformation of the optical mechanism will cause significant optical losses at connections. Previous studies have concentrated on improving the manufacturing process to obtain high-precision components. Although the precision can be controlled, misalignment may still occur owing to the contact stress on the connecting interface. This study used the finite-element method to simulate the contact status, and used MT-series connectors as examples. The connectors use a guide-pin structure to align the two contact surfaces, and use a spring or clip to maintain tight contact. Because the MT ferrules are made of plastic materials, they are softer than ceramic ones and the deformation strains are more significant. For the finite-element analysis, a solid model of the MT ferrule with a convex end, oblique angle, and fiber protrusion can be constructed according to the JIS C5981 standard. Results of a simulation, integrating the optimization technique and ANSYS software, showed that the fiber center displacement for the commonly used oblique PC connectors would be 0.9 μm. This will significantly affect the eccentricity tolerances for single-mode applications, because a maximum of 1.5 µm is acceptable. Using the resulting new standard (eccentricity <0.6 μm) to select ferrules, and making them into connectors, it is found that the insertion losses in 98% of connectors are less than 0.3 dB. This result satisfies the requirement for single-mode applications.