We have investigated the spin-orbit interaction in Strain-free Al0.1In0.9Sb/InAs0.1Sb0.9 quantum wells with different widths. The weak anti-localization (WAL) effect was clearly observed in the magneto-resistance for all samples. The inelastic scattering time is close to the T-1 law predicted theoretically for electron-electron interaction in the diffusive regime, and the spin-orbit scattering time is temperature independent as expected for the D'yakonov-Perel mechanism. The zero-field spin-splitting energies extracted from these observations were around 1.0 meV. The WAL behavior becomes distinct in samples with wider well width, and the zero-field spin-splitting energy slightly increases with well width. This seems to reflect increase of the carrier concentration. (C) 2008 Elsevier B.V. All rights reserved.