Two different constitutive models are employed to describe creep crack growth under small-scale creep conditions theoretically. The first model combines elastic/nonlinear viscous deformation fields \(\mathop \varepsilon \limits^ \cdot = \mathop \sigma \limits^ \cdot /E + A{\sigma ^n}\) with a critical-strain criterion, e = e c to be satisfied at a structural distance x c , ahead of the crack. The deformation fields are characterized by three singular fields — the remote elastic field, the HRR field in the creep zone, if one exists, and the Hui-Riedel (HR) field near the growing crack tip. At small stress intensity factors, crack growth is found to become irregular and to tend to instabilities. This is a consequence of the properties of the HR field.