Nanocavitation was detected for the first time in carbon black filled styrene-butadiene rubber (CB-SBR) under uniaxial loading by real time small-angle X-ray scattering (SAXS) using synchrotron X-ray radiation. A three phase model was developed to calculate the void volume fraction from the scattering invariant Q determined from the observed SAXS patterns. The normalized scattering invariant Q/Q(0), where Q(0) is the invariant before deformation, greatly increased above a critical extension ratio lambda(onset) which we attribute to the formation of nanovoids. Analysis of the 2D scattering patterns show that voids formed are 20-40 nm in size and elongated along the tensile direction. Cavities formed beyond lambda(onset) are smaller as lambda increases. Results from the scattering experiments are strongly supported by macroscopic volume change measurements on the samples under similar uniaxial strain. A nearly constant nanocavitation stress sigma(onset) (25 MPa) was observed when the filler volume fraction phi(CB) was larger than 14%. This value is much higher than that predicted based on the elastic instability of small voids in an unfilled elastomer and shows only a weak dependence on the cross-linking density v(C) in heavily cross-linked samples. An energy based cavitation criterion stressing the importance of confined domains between particles or clusters of particles was adopted and found to be consistent with the observed results. The nanocavities are thought to alter the local stress state and promote local shear motion of filler particles.