Radiography was performed to examine solidification and melting behaviors in hypereutectic cast iron specimens with / without Mg addition. The specimens without Mg addition showed that graphite particles exhibited spherical shape at the beginning and then transformed to the flake-like shape. Growth of γ-Fe dendrites was followed by nucleation of graphite particles ahead of the eutectic front. In contrast, growth of graphite particles and γ-Fe dendrites occurred nearly at the same time in the specimens with Mg addition and the spherical graphite particles were engulfed by γ-Fe or eutectic. The addition of 0.05mass%Mg significantly reduced the temperature range in which the graphite particles grew as the primary phase. Image-based analysis of melting behavior clearly proved that even 0.05mass% addition modified the phase equilibrium of the liquid, γ-Fe and graphite phases.