Absorption, photoluminescence, photoreflectance and transmission electron microscopy measurements have been performed on 2 and 3-monolayer thin InAs quantum wells(QW) (with 250Å wide GaAs barriers) grown on (511) GaAs substrate. For comparison, similar samples grown on (100) substrates have also been studied. (511)-grown thin quantum wells may have possible quantum wire configuration. However, polarization studies show a small anisotropic absorption from the (511) sample, which indicates that the optical property of the (511)-quantum well is different than either that of a quantum wire or that of a (l00)-grown quantum well structures. A theoretical calculation, making a unitary transformation of the valence band k•P Hamiltonian matrix to the (511) base, and using a perturbation method to determine the new wavefunctions, yield an anisotropic absorption comparable with the experimental result. We have also compared the transition energies from PL data with the calculated one using the conventional effective mass approximation.