Electron impact collision strengths are computed for Ar XII in a distorted wave approach and energy levels, oscillator strengths, spontaneous radiative decay rates for the collision target, as well as collisional high energy limits are obtained from an atomic structure calculation. The target expansion comprises the six spectroscopic configurations 2s 22p 3, 2s2p 4, 2p 5, 2s 22p 23s, 2s 22p 23p, and 2s 22p 23d, giving rise to 72 fine-structure levels in intermediate coupling. Collision strengths are calculated at five incident energies: 35, 70, 105, 140, and 175 Ry. They are complemented by Coulomb-Born limits towards infinite collision energy for forbidden transitions and line strengths for optically allowed transitions. Excitation rate coefficients are obtained as a function of electron temperature by assuming a Maxwellian electron velocity distribution. Using the excitation rate coefficients and the radiative transition rates, 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 = 6.4, corresponding to maximum abundance of Ar XII. Relative spectral line intensities are also calculated.