Nano-sized titanium dioxide (TiO₂) has wide industrial applications with considerable chance of exposure to humans and ecosystem. As a photocatalyst, nano-sized TiO₂ has been used in various environmental remediation applications implying its potential environmental impact. In order to better understand the interaction between nano-sized TiO₂ and phytoplankton in the aquatic environment, we first studied the adsorption of nano-sized TiO₂ (Degussa P25) onto the algae exemplified by Pseudokirchneriella subcapitata. P. subcapitata were exposed to TiO₂ in a series of concentrations and at various pH and ionic strength. The sedimentation experiments were conducted to determine the TiO₂ adsorption density using turbidity and dynamic light scattering measurements. Results show that the kinetics and the extent of TiO₂ adsorption are highly dependent on pH, which indicates the electrostatic nature of the interaction. The maximum adsorption density occurred (9x10⁵ particles per cell) at pH 5.5. As the ionic strength increased, the rate of adsorption increased but the saturation adsorption density remained relatively unchanged. The adsorption was multi-layered and the presence of isotherm plateau suggested that the adsorption energy between layers decreased toward the outer layer. The flocculation of algae was observed. At pH 4.6, the aggregate size reduced from hundred microns to a few microns as the number ratio of TiO₂ to algae increased from 3 x 10⁵ to 10⁶. The decrease in aggregate size can be explained by the increase in stability due to charge reversal on the algal surface brought by TiO₂ adsorption.