Three types of bimetallic AgAu nanoparticles, with mean size of 4-5 nm, Ag(core)Au(shell), Au(core)Ag(shell) and alloyed AgAu, have been synthesized using an inverse micelle method. To image these small size nanoparticles, quantitative high angle annular dark field imaging using scanning transmission electron microscopy was successfully applied. Our results show that good control of nanoparticle size dispersion and composition modulation was achieved. Optical properties of the nanoparticles are correlated with direct internal structure analysis. The structural stability is discussed, based on thermodynamic considerations.