Nano and submicron titanium dioxides are widely used as ingredients of sun screen. Thus, its reproductive and developmental toxicities in humans are of concern. We used two kinds of nano titanium dioxides, involving different production methods and surface coatings. The embryotoxicities of these nano titanium dioxides were determined based on inhibitory effects on the cardiomyocyte differentiation of mouse Embryonic Stem cells. As ultrafine titanium dioxide, titanium dioxide (particle diameter, 0.03-0.05 μm) produced by the sintering method (T1) and that (particle diame-ter, 0.01-0.02 μm) produced by the wet method (T2) were used. T2 may cause little developmental toxicity because it has no potent inhibitory effects on cardiomyocyte differentiation. However, T1 should be further investigated.