Minimally invasive robotic surgery is recently attracting much attention, because it can alleviate patients'' pain, bleeding, and long-term recovery, etc. If the robotic surgery end has a haptic device equipped with a 6-dof force sensor, surgeons can easily control their operating grasping forces. In an effort to develop a 6-dof optical sensing device with a FBG (Fiber Bragg Grating) sensor which is applicable to detect forces acting on the end-effectors of a minimally invasive surgical robot, a new design analysis method has been developed. The sensing device attached to an ed-effector consists of a cylindrical frame structure FBG optical fiber surrounding the frame diagonally. As the frame deforms subjected to forces and moments, the attached FBG fiber with 2000 micro-strain of pretension aldo elongates or contracts. Deformations of the frame was calculated by the finite element method subjected to x-, y-, and z-direction forces and three moments about x, y, and z axes, respectively. Displacements of the FBG fiber regarding to six load-types could be then extracted from frame deformation data and converted to changes in wavelength on the basis of an equation of optical fiber reflected wavelength and FBG strain. Using the developed methos, various wavelengths were calculated as three forces changed from 0 to 10 N and three moments varied 0 to 100 Nmm. To validate accuracy of the analytic design method, a system of the 6-dof FBG sensor with a frame was fabricated and tested under the loads used for analyses. The changes in wavelength were measured by an optical spectrum analyzer in the experiment. Analytical and experimental results showed a good correlation with 9.7% error. The sensitivities of the theoretical and analytical models were very similar.