This paper applies the particle image velocimetry (PIV) to measure the laminar boundary layer thickness of TiO2-water nanofluid free convection over the vertical and horizontal flat plates with constant heat flux. The results are presented for two values of heat flux strength equal to 3000 and 7000 w/m(2). The effects of nanoparticles and heat flux strength on the boundary layer thickness and velocity profiles are investigated in details. Finally, a comparison is performed between the experimental, theoretical and numerical results for different conditions. The obtained results revealed that the velocity boundary layer decreases by adding the nanoparticles to the base fluid. These reductions, at phi = 0.1%, are in the vicinity of 7.5% and 5.2% for q '' = 3000 and 7000 wm(-2), respectively. Also, the maximum reductions in the vertical velocity component by adding the nanoparticles at phi = 0.1% are in the vicinity of 4% and 3.3% for q '' = 3000 and 7000 wm(-2), respectively. (C) 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.