A systematic shell model calculation on the structure of light tin isotopes is presented with a monopole-optimized effective interaction. We started with the realistic CD-Bonn potential. The unknown single-particle energies and the T = 1 monopole interactions are determined by fitting to the binding energies of the yrast states of nuclei Sn102-132. The mean deviation from experimental data is around 120 keV. In particular, we analyzed the origin of the spin inversion between Sn-101 and Sn-103.