The effect of micronization on sun protection factor (SPF) were tested on two types of titanium dioxide (TiO2) with primary particle sizes of 20 nm and 170 nm. Oil/water creams with 5%, 10%, and 20% concentrations of each type of TiO2 were prepared,and SPF was measured using both in vitro and in vivo methods. In vitro analysis demonstrated that submicron-sized TiO2 cream had a lower SPF value than nanosized TiO2 formulations of the same concentration. In vivo experiments confirmed this result, and a strong correlation between in vitro and in vivo measurements was observed. Furthermore, the SPF values of nanosized TiO2 sunscreen wereconcentration-dependent in the range of 5% to 20%. Scanning electron microscopy results indicate that the higher SPF of nanosized TiO2 formulations may be due to the formation of multilayer agglomerates by small particles at nano-scales,leading to a reduced void space between particles and a more efficient barrier to protect skin from sunlight.