The flux cored wires with different rare earth oxide additions for hardfacing the workpieces of medium-high carbon steel were developed. The microstructure of the hardfacing layer was observed using the optical microscopy. The average dimension of primary austenite grains in hardfacing layer was measured by image analyzer. The primary austenite grain growth activation energy and index were calculated according to Sellars’s mode and Beck formula, respectively. Moreover, the effect of rare earth oxide on the growth dynamics of primary austenite grain was analyzed, and then discussed with the misfit theory. The experimental results showed that, by adding rare earth oxide, the average dimension of primary austenite grains in hardfacing layer of medium-high carbon steel decreased, and it was the smallest when the addition of rare earth oxide was 5.17 wt.%. Meanwhile, at this rare earth oxide addition, the primary austenite grain growth activating energy in hardfacing layer was the largest, while its index was the smallest. The calculated results indicated that the primary austenite grain could be refined because LaAlO3 as heterogeneous nuclei of γ-Fe was the most effective. The flux cored wires with different rare earth oxide additions for hardfacing the workpieces of medium-high carbon steel were developed. The microstructure of the hardfacing layer was observed using the optical microscopy. The average dimension of primary austenite grains in hardfacing layer was measured by image analyzer. The primary austenite grain growth activation energy and index were calculated under Sellars’s mode and Beck formula, respectively. respectively, the effect of rare earth oxide on the growth dynamics of primary austenite grain was analyzed, and then discussed with the misfit theory. The experimental results showed that, by adding rare earth oxide, the average dimension of primary austenite grains in hardfacing layer of medium-high carbon steel decreased, and it was the smallest when the addition of rare earth oxide was 5.17 wt.%. Meanwhile, at this rare earth oxide addition, the primary austenite grain growth activating energy in hardfacing layer was the largest, while its index was the smallest. The calculated results indicated that the primary austenite grain could be refined because LaAlO3 as heterogeneous nuclei of γ-Fe was the most effective.