The antenna geometry strategy for direction finding (DF) with multiple-input multiple-output (MIMO) radars is studied. One case, usually encountered is practical applications, is considered. For a directional antenna geometry with a prior direction, the trace-optimal (TO) criterion (minimizing the trace) on the average Cram´er-Rao bound (CRB) matrix is employed. A qualitative explanation for antenna geometry is provided, which is a combinatorial optimization problem. In the numerical example section, it is shown that the antenna geometries, designed by the proposed strategy, outperform the representative DF antenna geometries.