The magnetic properties of single crystalline hcp Co nanowire (NW) assemblies with different packing densities are investigated. The compacted assemblies exhibit an unusual increase in coercivity (HC) from 5.5 kOe to 6.1 kOe with the increase of the packing density from 2.1 to 4.5 g/cm3. While Hc decreases with further increase in packing density. A similar trend in Hc value variation with density has also been noticed at different temperatures. The initial increase in HC values is ascribed to magnetostatic interaction between the nanowires, which became stronger with the increased packing density. However, above the density of 4.5 g/cm3, the Hc decreases due to the proximity effect as shown in the δM plot. The δM plot shows a negative peak at high field and it becomes prominent with the increased density. Moreover, it has been found that the variation of HC with the packing density is also related to diameter of Co NWs. A larger diameter of the nanowires gives a negative dependence of the Hc value with the density in the whole investigated region.