Numerical simulation technology was investigated by using the optical software GLAD to design Gradient Index (GRIN) lens based miniature probes for imaging of Optical Coherence Tomography (OCT). Firstly, the basic features of the GRIN lens were overviewed,and design methods for GRIN lens based optical probes were discussed. Then, the probe model consisting of a single mode fiber, a glass rod spacer and a GRIN lens were simulated. The simulating results show that the numerical simulation technique using GLAD can provide an intuitive and effective method for design of miniaturized probes and verification of their optical performance. In addition, the spacer can improve the optical properties of the GRIN lens based optical probes for there exists a suitable range of the spacer lengths in the glass rod.It shows that the working distance of the probe will be greater than 1.0 mm and the focus spot size less than 40 μm when the constant length of GRIN lens is to be 0.1 mm and the spacer length range from 0.8 to 1.1 mm.