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Mechanical properties in tension of cobalt have been studied in a wide range of grain size d using thickness t of samples from 125 to 700 μm. The Hall and Petch (HP) relationship indicates that the yield stress strongly varies for t/d below a critical value around 14, highlighting the existence of a multicrystalline regime. The behavior becomes almost single crystalline when t/d is lower than 2.5, with a very small HP slope. The reduction of the t/d ratio has also a large influence on the work hardening of cobalt, especially in the beginning of plasticity, which mainly corresponds to the stage of basal slip mechanisms. However, these size effects tend to vanish when twinning becomes predominant. Nanoindentation measurements and application of a simple composite model show that the appearance of the multicrystalline regime is closely related to surface effects that develop through the thickness of the specimens. In addition, the Kocks - Mecking work hardening model predicts a significant increase of the average free path of dislocations in the basal slip stage. A quantitative analysis of dislocation pile-ups that develop during this stage supports the hypothesis that the mechanical softening observed in cobalt multicrystal is related to a higher activity of dislocations, which can escape more easily on free surfaces. When twinning becomes the main deformation mechanism, dislocation glide is restricted which reduced, in turn, the surface effect. Consequently, size effects gradually disappear with hardening.