With the top-surface scattering considered, an improved model was proposed in this paper to predict the electron mean free path (EMFP) of a rectangular nanowire. The proposal of this model was based on the statistic simulation and the related fittings. The model is generally suitable for predicting EMFPs of rectangular nanowires, nanofilms and nanoparticles. The top-surface scattering was studied based on the model. And some issues about the Cu rectangular nanowires, rather than the widely studied sidewall-surface scattering and the grain-boundary scattering, were discussed. It is predicted that the top-surface scattering would be important if the length of the nanowire is close to the size of the cross-section. If short nanowires, with the same cross-sectional area but different lengths, are prepared and tested, the effect of top-surface scattering may be observable. Though a good resistivity agreement between the model and the experiment can be reached, fitting parameters of the model may be quite different for different authors, even with the same nanowire considered.