The differential image motion monitor (DIMM) is a widely used instrument for measuring atmospheric coherence length. We studied the feasibility to miniaturize the classic DIMM. By adopting more accurate formulas, the geometry of the classic DIMM can be altered mildly. A mini-DIMM can be developed under this new geometry condition. For example, a mini-DIMM may consist of two 6-cm apertures, located close to each other. We present the validation experiment where astronomical seeing was measured simultaneously by a mini-DIMM and a standard DIMM. Comparisons with a standard DIMM show good agreement between the two instruments.