jats:titleAbstract</jats:title>jats:pWe study the role of grain size in the rheological behavior of calcite aggregates in the semi‐brittle regime. We conduct compressive triaxial deformation tests on three rocks, Solnhofen limestone, Carrara marble and Wombeyan marble, with grain sizes of 5–10, 200, and 2 mm, respectively, at pressures in the range 200–800 MPa and temperatures in the range 20–400°C. At all conditions, both strength and hardening rate increase with decreasing grain size. Flow stress scales with the inverse of grain size to a power between 1/3 and 2/3. For a given pressure and temperature, the typical strength of Solnhofen limestone (grain size 5–10 μm), is greater than that of Wombeyan marble, (grain size of around 2 mm), by about 200–300 MPa (i.e., around a factor of 2). Hardening rate decreases linearly with the logarithm of grain size. In situ ultrasonic monitoring reveals that jats:italicP</jats:italic>‐wave speed tends to decrease with increasing strain, and that this decrease is more marked at room temperature than at 200 and 400°C. The decrease in wave speed is consistent with microcracking, which is more prevalent at low temperature and low pressure. Microstructural observations reveal high twin densities in all deformed samples. Twin density increases with stress, consistent with previous datasets. Spatial distributions of intragranular misorientation indicate that twins are sometimes obstacles to dislocation motion, but this effect is not ubiquitous. Computed slip‐transfer statistics indicate that twins are typically weaker barriers to dislocation glide than grain boundaries, so that their effect on dislocation accumulation and hardening rates is likely smaller than the effect of grain size.</jats:p>