In this study we explored the rheological characteristics of Carbopol C934 gel (polyacrylic acid) containing SUS304 spherical nanoparticles of 100 nm as a simulant of gel propellants containing metal fuels. In comparison with the pure Carbopol gel, the SUS nanoparticle filled Carbopol gel exhibited stronger shear thinning and higher yield stress. As the concentration of nanoparticles increased yield stress increased, but viscosity and storage modulus increased first and then decreased abruptly beyond the critical limit. Also as the concentration of nanoparticles increased there was a transition in material characteristics from the ductile type to the brittle type, which means that highly filled Carbopol gels lost the structure almost instantaneously as the imposed stress was larger than the yield stress, while Carbopol gels of low particle loading sustained the structure even after the imposed stress was larger than the yield stress. The cryogenic scanning electron microscopy analysis revealed that the network structure changed abruptly when the rheological properties changed abruptly. The change in gel structure is attributed to the nanoparticles that compete with Carbopol chains in forming networks. The abrupt change in gel structure with the addition of particles beyond the critical limit should be an exclusive phenomenon of nanoparticles. (C) 2011 The Society of Rheology. [DOI: 10.1122/1.3538092]