A temperature-insensitive fiber optic sensor is devised to measure the contact force at the tool tip of transluminal surgery. The force sensor is made with a pair of fiber Bragg gratings (FBGs), which are multiplexed on a single mode fiber and sit in the state of tension inside two tubes, a nitinol tube and a stainless steel tube. Their material stiffness difference leads to the sensitivity difference to force, which is analyzed by the finite element method and is validated by the sensor's calibration. A decoupled model based on sensitivity difference is used to compute the force from the wavelength shifts of the two FBGs, overcoming the influence of temperature change during force measurement. Comparative experiments have been done to assess the model, and we find that the force computed by the compensation model is much more accurate than the non-compensation model. The devised sensor can be used to accurately measure force under varying temperature.