C36 Laves phase with a nominal composition of (Mg,Al)2Ca and a hexagonal structure can form via eutectic reaction under non-equilibrium solidification condition in Mg-Al-Ca based alloy. Furthermore, different types of defects (i.e. stacking fault and dislocation) was observed within C36 Laves phase. However, the distribution of Mg within C36 Laves phase and the segregation of Mg along these defects still remains to be explored, especially at atomic scale. In this contribution, atomic TEM, APT and DFT investigations on the Mg distribution within C36 Laves phase and the segregation of Mg along defects in Mg-3Al-1Ca alloy will be presented, with an aim to elucidate the role of Mg during the formation of C36 Laves phase. Mg-rich particles with an oval morphology, a size of 20 nm and an enrichment of trace Al (8 at. %) and Ca (4 at. %) were observed within C36 Laves phase. While, C36 Laves phase itself also contains a high amount of Mg and Al with a rate of (Mg,Al)3Ca, which is much higher than the nominal composition (Mg,Al)2Ca. The presence of Mg-rich particles within C36 Laves phase can be attributed to the divorced eutectic reaction, which can be further supported by the experimental observations that Mg-rich particles and C36 Laves phase keep a defined orientation relationship (i.e. [112 ̅0]Mg//[112 ̅0]C36, (0001)Mg//(0001)C36) and the composition of Mg-rich particles is very close to the Mg matrix. The segregation of Mg along stacking faults was also observed, which can be further supported by DFT simulation. After homogenization at 500 °C for 20 h, C36 Laves phase has transformed into C15 (Al2Ca) Laves phase via the diffusion of Mg out of C36 Laves phase. After rolling at 300 °C up to 6 passes, no Mg-rich particles but the segregation of Mg along dislocations and/or stacking faults was observed. This presentation shows the Mg distribution within Ca-rich Laves phases and the segregation of Mg along defects, which can provide helpful insights in controlling the formation of Ca-rich Laves phases in Mg-Al-Ca based alloy.