We demonstrate molecule-specific third-harmonic-generation (THG) microscopy by using silver nanoparticles as THG contrast agents. Through matching surface plasmon wavelength to THG wavelength, strong contrast can be provided by silver nanoparticles under THG microscopy. By conjugating anti-her2 antibodies with silver nanoparticles, the expression of the Her2/neu oncogene in the cancer cell membranes is successfully imaged under THG modality for the first time.