Electron impact collision strengths, energy levels, oscillator strengths and spontaneous radiative decay rates are calculated for Si XI. The configurations used are 2s 2, 2s2p, 2p 2, 2l3l', 2l4l' and 2s5l', with l = s,p and l' = s,p,d giving rise to 92 fine-structure levels in intermediate coupling. Collision strengths are calculated at five incident energies (35, 70, 105, 140, and 175 Ry) in the distorted wave approximation. Excitation rate coefficients are calculated as a function of electron temperature by assuming a Maxwellian electron velocity distribution. Using the excitation rate coefficients and the radiative transition rates of the present work, and R-Matrix results for the 2s 2, 2s2p, 2p 2 configurations available in the literature, statistical equilibrium equations for level populations are solved at electron densities covering the range of 10 8-10 14 cm -3 at an electron temperature of log Te(K) = 6.2, corresponding to the maximum abundance of Si XI. Spectral line intensities are calculated, and their diagnostic relevance is discussed. This dataset will be made available in the next version of the CHIANTI database.