This study investigated the influence of cooling rate and Sn addition on the microstructure formation of as-cast GCD700 spheroidal graphite cast irons. Changes in cooling rate manifested as step cast thickness differences. Optical microstructures of as-cast GCD700 alloys revealed α-ferrite and pearlite and dispersed graphite nodules. In all examined thicknesses without Sn, the α-ferrite, rather than the pearlite, surrounding graphite nodules appeared to dominate microstructures, and the graphite looked well rounded, whereas microstructure containing 0.09% Sn had a significantly expanded pearlite area. Image analysis showed numbers of graphite nodules increased only on decreasing cast thickness. However, the phase fractions of ferrite and pearlite were not dependent on thickness. For samples containing Sn, pearlite fractions significantly increased with Sn content. Thermodynamic calculations and scanning electron microscopy-based microstructural analysis confirmed that the Sn contents examined had no significant effect on phase formation, Sn segregation, or the relationships between ferrite and Fe3C orientations in pearlite.