Thermal properties of a series of Co $eta$-diketonates have been systematically investigated and it is found that tris(3,5-heptanedionato) cobalt(III) ($Co(hd)_3$) with the lowest melting point among them can be a better precursor than tris(2,2,6,6-tetramethyl-3,5-heptanedionato cobalt(III) ($Co(tmhd)_3$), one of the most popular precursors to date, under suitable conditions. Isothermal TGA study shows that $Co(hd)_3$ would work better at higher temperature, while $Co(dmhd)_3$ would be a better precursor at lower temperature.