Au and Ag nanoparticles have strong light absorption band in a visible spectral region because of a collective excitation of free electrons in their surface, whose quantization is surface plasmon. The spectral position and width of surface plasmon are modified by the size, environment, shape, and structure of nanoparticles. In this work, spherical Au/Ag core-shells were used to investigate their nonlinear optical transmittance and cubic nonlinearity by Z-scan and degenerate four-wave mixing techniques. The core-shells were uniformly distributed in distilled water. The average size and thickness of core (Au) and shell (Ag) were ~13.9 nm and ~9 nm, respectively. The absorption peak was around 410 nm, which can be attributed to surface plasmon resonance. The excitation source was a 6 ns pulsed Nd:YAG laser at 532 nm of wavelength with 10 Hz of repetition rate. The laser peak intensity was increased from ~8 MW/cm² – 7.5 GW/cm² for optical power limiting and cubic optical nonlinearity measurement of Au/Ag core-shells. The nonlinear transmittance using intensity dependent measurement revealed that the optical power limiting threshold was ~6 GW/cm² at near valley of open Z-scan. The second hyperpolarizability of Au/Ag core-shells increased as the shell thickness was increased from ~0.6 to 9.0 nm. The optical cubic nonlinearity of Au/Ag core-shells are mainly from anharmoinc oscillations of free electrons in the conduction band, interband transition of bound electrons, and thermalization of coreshells in aqueous solution. This work at Hampton University was supported by Army Research Office (W911NF-07-1-0608) and National Science Foundation (HRD-0734635, HRD-0630372, ESI-0426328/002, and EEC-0532472).