The optical modulation format conversion is a key technique in the future all-optical communication. An optical modulation format conversion technique from non-return-to-zero (NRZ) signal to return-to-zero (RZ) signal is presented, based on the cross-phase modulation (XPM) effect in a silicon micro-ring resonator. When a NRZ signal acting as the probe and a periodic pulse train as the pump are launched into the silicon micro-ring resonator synchronously, the power change of the pump corresponding to the rising or falling edge leads to negative or positive chirp of the probe frequency caused by XPM. The converted RZ signal can be extracted by utilizing appropriate optical band pass filters (OBPFs). The dependence of the quality of converted RZ signal on the power and pulse-width of pump is studied, as well as that on the wavelength detuning of an OBPF to the probe. It is shown that the optical modulation format conversion can be achieved in a chip by the XPM nonlinear effect of the silicon micro-ring resonator.