Tensile specimens of Type 316L stainless steel, containing boron and having grain sizes in the range 3.1 to 103.8 μm, were deformed at 24, 400 and 700°C at a constant strain rate of 1 × 10-4 s-1. The flow stress (σ) varies with strain (ε) and grain size (d) according to the Hall-Petch (H-P) type relationship σ(ε) = a εa + b εmd-0.5 with the H-P parameters (σ0 and k) interrelated through the constants a, n and b, m. The presence of boron affects the flow stress and changes the H-P parameters, depending on test temperature. The dislocation density increases with temperature due to dynamic strain ageing, which leads to some anomalous effects of grain size at 700°C. © 1997 Acta Metallurgica Inc.