Abstract: The covariance matrix structure of clutter plus jamming is analyzed for side-look airborne MIMO radar, and the upper bound on its rank is derived to equal the summation of clutter and jamming rank subtracting the jammer number. According to this, a certain range of jammer number is attained. If the number of jammers is in this range, the clutter plus jamming covariance matrix is full rank for SIMO radar, whereas rank deficiency for MIMO radar. As a result, the performance of SIMO radar deteriorates severely under this condition, however, the performance of MIMO radar is much better, due to sufficient degrees of freedom to suppress clutter and jamming for MIMO radar. The simulations validate the above conclusion.