The misfit stress induced by a lattice mismatch is determined by a curvature analysis performed by Transmission Electron Microscopy on plan view samples. The different samples examined consist of Ga$_{1-x}$InxAs layers grown by molecular beam epitaxy on a (100) GaAs substrate, with a nominal mismatch of 0.7 and 1.4%. The layer thicknesses are lower than the critical thickness for plastic relaxation. The curvature radius as well as the substrate thickness are determined directly by bend contour analysis. For each sample, the measurement was performed on different areas corresponding to different substrate thicknesses. The experimental value of the in-plane component of the stress is deduced by applying the Stoney's formula over the whole range of substrate thickness. The accuracy of the method is better than 15%. Experimental values of the misfit stress are ten to fifty per cent lower than the theoretical value calculated for pseudomorphic layers. This discrepancy is discussed in terms of partial relaxation.