According to the creep test principle of friction of V-belt driving, the power transferred is closely related to the belt sliding angle. The deficiency of conventional formula for calculating the sliding angles is analysed, and an optimized formula is introduced. Tests that fiber optic sensors are used to measure the belt sliding angles are carried out. Data acquisition is accomplished by high speed PCI interface card on computers in order to measure the magnitude of transient sliding angles. Then the sliding angle changes are correlated to the corresponding load conditions and an optimized formula for V-belt sliding angle calculation is obtained. It is found that the sliding angle calculated by the new formula is larger than by the traditional formula, but more approximate to the experimental values in both trend and magnitude.