The magnetic moment of spin-1/2 fermion with mass m and charge e is predicted as μ= e/ 2 m from the Dirac equation. The fermion precesses in magnetic field (B) with angular velocity ωp= eB/ m due to the magnetic moment. On the other hand, the cyclotron frequency of the fermion moving in the same magnetic field is ωc= eB/ m. Since ωc= ωp, the helicity (spin component of the momentum direction) of the fermion conserves in any magnetic field. However, it is observed that there is actually a slight difference between ωc and ωp. The origin of the difference is that the higher order diagrams add very small spin-flipping transition amplitude to the original one and change ωp. This effect is important because we can look new physics out through the additional transition amplitude.