The quantum interference is studied in a spontaneous emission of a four-level atom driven by three coherent fields. Deriving the equation of motion in the interaction picture we obtain the susceptibility. It is shown the susceptibility is different from the conventional nonlinear optics. The atom exhibits strong nonlinear effects when the spontaneous is at resonance, though it is weak. We can apply this new phenomenon to many fields such as making quantum device, detecting weak signal and processing quantum information.