A new layered metal–organic hybrid compound, namely, [Co3(μ3-OH)2(BTP)2] (1; BTP=4-(3-bromothienyl)phosphonate), is reported. The inorganic layer can be viewed as a pseudo-Kagomé lattice composed of corner-sharing irregular triangles of Co3(μ3-OH), with the cavities filled with the PO3 groups. The interlayer space is occupied by the 3-bromothienyl groups of BTP2−. The bulk sample of compound 1 experiences a long-range ferromagnetic ordering below 30.5 K, with a coercivity (Hc) of 5.04 kOe at 5 K. A systematic study on the size-dependent magnetic coercivity of 1 reveals that the coercivity of 1 increases with reduced particle size from the micrometer to the nanometer scale. When the particle size is about 50–200 nm, the coercivity reaches 24.2 kOe at 5 K. The results demonstrate that compound 1 can vary from a soft magnet to one of the hardest molecule-based magnets, simply by reducing the particle size to nanoscale region.