A slip sensing unit based on cantilever structure is developed. The Bragg wavelength shifts of three fiber Bragg gratings and variance of the wavelength difference are used to predict whether slip happens or not, so that the fluctuation of the contact force, the sliding rate and direction can be detected. The experimental results show that the proposed slip sensing unit can detect micro slip within the range of 0.2~1 N of the contact force, whose minimum sensitivity is 3.8 pm/mm. The average relative errors between the experimental value and the theoretical value are 6%, 14% and 6.6% for the slip sensitivity, the slip speed coefficient and the contact force, respectively.