This paper has investigated the deformation behaviours and the size effect on the yielding of Mg24Y5 using nanoindentation, micro-pillar compression and in-situ TEM compression. The elastic modulus and hardness are measured to be 54.2 GPa and 3.2 GPa, respectively. The displacement discontinuities (i.e. pop-in) on the load-displacement curves were observed both in nanoindentation and in-situ TEM compression. It has been found that the primary slip system activated in the tests was regardless of the sample size and sample surface acted as the initial dislocation source. Molecular dynamics simulation showed that shear band was found to act as dislocation source with a configuration similar to Frank-Reed source. The deformation microstructure was studied using transmission electron microscopy. The sample size effect on the critical resolved shear stress measured from Mg24Y5 follows a power-law relationship with the estimated critical resolved shear stress for bulk sample of 265 MPa. It is suggesting that the observed size effect is due to the different dislocation source lengths which led to different stresses required for nucleation dislocations.