The K447A superalloy was successfully prepared by Microcast-X (MX) fine grain casting process in this work. After heat treatment, the mechanical properties and microstructure of the MX K447A alloy were analysed and compared with the conventional casting alloy. In addition, the deformation mechanism associated with substructure evolution was also analysed.The yield and tensile strength of MX K447A are 1022MPa and 1219MPa respectively, which are increased by 200MPa and 160MPa, compared with the conventional ones. By microstructure analysis, the average grain size of MX K447A is 54μm, which is reduced by about two orders of the conventional one. Apart from that, the volume fraction of γ' has also increased from 34.2% to 51.6%, owing to elimination of Ti and Al segregation. By quantitative analysis with strengthening formula, the main improvement of strength can not only be attributed to grain refinement but also the increase of precipitates volume fraction, which account for the increase of yield strength by 86MPa and 88MPa respectively. In addition, the substructure results show that plastic deformation occurs at the grain boundaries firstly and then expands into the grains. Within the grain, dislocations bypass primary γ’ via Orowan looping at the beginning. Along with deformation, crossed slip bands and low-energy dislocation cells are formed to relax stress and coordinate deformation.