Nanoscale lateral variations in the stress field of undulated Si_0.7Ge_0.3/Si(100) films have been experimentally studied via in situ transmission electron microscopy annealing and through finite element calculations. When annealed at ~480 °C, misfit dislocations in a 30 nm film (having surface undulations of ~70 nm wavelength and ~3 nm amplitude) propagated at 80 nm s⁻¹ average speed but with periodic variations from 0–30 nm s⁻¹ at the peaks of the undulations to 160–240 nm s⁻¹ at the troughs. A 2.0 GPa average film stress with variations from 3.2 to 4.4 GPa at the troughs to 0.7–1.2 GPa at the peaks is inferred from the observed dislocation velocities. These stress variations are significantly higher than those calculated from a finite element model of Si_0.7Ge_0.3/Si with the same surface geometry. Using standard models of dislocation kink dynamics, we have calculated how the effect of high stresses at the undulation troughs would be expected to enhance kink nucleation rates, and have found good agreement between our models and the experimentally observed range of dislocation velocities. These observations demonstrate the potential of probing the nanoscale structure in thin films through local variations of dislocation velocities.