The cross Kerr nonlinearity effect in RY-type four-level atomic system is theoretically studied. Using density-matrix equations and perturbative iterative method, dependence of real part of third-order susceptibility on variations of additional coherent coupling field intensity and nonradiative decay rate between two ground states of atoms are derived. It is found that, by decreasing the nonradiative decay rate between the two ground states and increasing the coupling field intensity, the enhanced cross Kerr nonlinearity with vanishing absorption can be obtained at two-photon Raman resonance. The result is useful in application of all-optical switch.