The three well-known parameters of motion (magnetic momentum, energy and canonical toroidal momentum) are utilized to develop a guiding center orbit code, GCORBIT. The effect of poroidal field ripple and toroidal angle scatter is considered in the GCORBIT. It is applied to compute guiding center orbit of charged particle in axisymmetric tokamak, the loss region of velocity space and the loss fraction of fast ions.To compare achieved results, the loss region of velocity space by GCORBIT includes many effects: non-closed orbit of divertor, first orbit loss by guiding-center orbit being non-closed or intersecting with any material limiters, trapping by the poroidal field ripple in local magnetic wells, ripple random diffusion and toroidal angle scatter enabled by velocity space discontinuous confinement region. The GCORBIT is now being used to analyze the confinement of fast ions in EAST Tokamak. Our numerical calculations are focused on the loss of fast ions.