Abstract: An investigation is made into the effect of water hardness on fine coal flotation and the law governing the variation of performance of foaming agent and coal surface wettability when water for flotation use varies in hardness. The study is made through adding calcium chloride in deionized water. As evidenced by test result,water hardness exerts a great bearing on yield and ash of flotation concentrate which tend to first increase and then decrease with increase of water hardness; the surface tension of the water solution of foaming agent is dominantly governed by water with a hardness of ＜ 700 mg/L,and the surface tension is observed to first decrease and then increase with increase of water hardness; and the water hardness produces a principal effect on surface wettability of coal particles at a hardness value of＞ 700 mg/L,and at a hardness of ＞ 900 mg/L,the extra pure coal and gangue particles show only minor difference in surface wettability,causing,therefore,a poor flotation selectivity.