Viscoelastic surfactant (VES) fluids are preferred for many applications in the oil industry. Their viscoelastic behavior is due to the overlap and entanglement of very long wormlike micelles. The growth of these wormlike micelles depends on the charge of the head group, salt concentration, temperature, and the presence of other interacting components. The problem with these fluids is that they are expensive and used at temperatures less than 200°F. The viscoelasticity of nanoparticle-networked VES fluid systems were analyzed in an HP/HT viscometer. A series of rheology experiments have been performed by using 2-4 vol% amidoamine oxide surfactant in 13 to 14.2 ppg CaBr2 brines and 10.8 to 11.6 ppg CaCl2 brines at different temperatures up to 275°F and a shear rate of 10 s-1. The nanoparticles evaluated were MgO and ZnO at 6 pptg concentration. In addition, the effect of different nanoparticle concentrations (0.5 to 8 pptg) and micron size particles on the viscosity of VES fluid was investigated. The oscillatory shear rate sweep (100 to 1 s-1) was performed from 100 to 250°F. The effect of fish oil as an internal breaker on the viscosity of VES micelles was examined. This study showed that the addition of nanoparticles improved the thermal stability of VES micellar structures in CaBr2 and CaCl2 brines up to 275°F and showed an improved viscosity yield at different shear rates. Micro- and nanoparticles have potential to improve the viscosity of VES fluids. Lab tests show that for VES micellar systems without nanoparticles, the dominant factor is the storage modulus but when nanoparticles are added to the system at 275°F the loss modulus becomes the dominant factor. These positive effects of nanoparticles on VES fluid characteristics suggest that these particles can reduce treatment cost and will exceed temperature range to 275°F. With this work, we hope to have better understanding of nanoparticle/viscoelastic surfactant interaction.