We have studied the photoreflectance spectra at 300 K from a series of strained In1−xAlxAs/InP (0.42<x<0.57) strained structures grown by molecular beam epitaxy. From the observed Franz-Keldysh Oscillation we evaluate the built-in de electric field and hence the surface potential under different strain. We found that the surface Fermi level is not pinned at midgap under different strainwhich results in contrast to AIGaAs and GaAs. In addition, from the observed dependence of the built-in electric field Fdc and surface potential barrier Vm on the top layer thickness, we conclude that the surface states are distributed over two separate regions within the energy band gap under different strain and the densities of the surface states are as low as (2,71α0.05)x 1011 cm−2 for the distribution near the conduction band and (4.29α0.05)x1011 cm-2 for the distribution near the valence band. The Fermi level is weakly pinned while the top layer thickness is within the characteristic region of each sample.