E-twinning is a common plastic deformation mechanism in calcite. Previous experiments have shown that temperature, strain rate and confining pressure have negligible effects on twinning activation, the latter being mainly dependent on differential stresses. Temperature is however reported to control the aspect of twin lamellae, with thickness exceeding 2-5 µm mostly at temperature above 200 °C. The critical resolved shear stress (CRSS) required for activation of twinning has been shown to be dependent on grain size and to be subjected to strain hardening: its value increases with the amount of strain accommodated and decreases with increasing grain size. This CRSS value may obey the Hall-Petch relation, but due to sparse experimental data the actual evolution of the CRSS with grain size and strain still remains a matter of debate. Numerous experiments were carried out at high temperature to investigate gliding systems in calcite, but just few data are available on the plastic behavior of calcite crystals at low temperature, despite the fact that the latter may help understand deformation processes of carbonate rocks in sedimentary basins. New mechanical tests were carried out at room temperature on unconfined single crystals of calcite, with different sizes and crystallographic orientations. Uniaxial deformation was performed at controlled displacement rate, meanwhile the sample surface was monitored using optical microscopy (reflected light) and high resolution CCD camera. The retrieved macroscopic stress-strain behavior of the crystals was correlated with the surface observations of the deformation process.