In order to improve the coupling efficiency between fibers and optical devices, a new thermally expanded core fiber collimator is designed. It consists of thermally expanded core (TEC) fiber and gradient index (GRIN) lens. The relation between TEC fiber collimator and its optical characteristics is analyzed. The coupling insertion losses caused by three kinds of misalignments are calculated. The results show that the coupling characteristics of TEC fiber collimator is related to the GRIN lens parameters, the lateral and angular coupling tolerances have been greatly improved compared with single-mode fiber collimators and the loss of axial offset is higher than that of single-mode fiber collimators. We fabricate the TEC fiber collimator by using the TEC fiber with 15.4 μm mode-field radius and the GRIN lens with 0.295 mm-1 gradient constant. The coupling insertion losses measured are consistent well with the theoretical results. The TEC fiber collimator can be used for long working-distance optical coupling and rotary connection devices.