The optimal alloy design and microstructure for the high temperature (around 800oC) constrained cyclic thermal fatigue applications in thin-section ductile irons have been established previously by the authors. This study intends to investigate the effect of graphite morphology on the thermal fatigue property of thin-section (2mm ~ 6mm) graphitic cast irons. The results show that to produce carbide-free spheroidal graphite irons in relatively thin sections of 2-mm or 3-mm in the as cast conditions is feasible. However, for compacted graphite cast irons, graphite particles largely in nodular form always occurs in rather thin sections, and acceptable compacted graphite structure can only be obtained when the section thickness exceeds about 6-mm. Regarding the constrained cyclic thermal fatigue property (20 ~ 800oC), cast irons with spheroidal graphite exhibit the best thermal fatigue life, which is followed by irons with compacted graphite, and then flake graphite cast irons, even though flake graphite cast irons show least swelling or distortion after cyclic thermal fatigue test. Furthermore, adding some 0.5%Mo to the irons significantly improves the thermal fatigue property, in some cases even by a factor of 2, implying that the role of Mo outweighs the influence of graphite structure in promoting thermal fatigue property.