NOVELTY - A coolant composition comprises fuel cell unit coolant containing aliphatic alcohol having unsaturated bond. The aliphatic alcohol is chosen from allyl alcohol, 2-butene-1-ol, 3-butene-1-ol, 1-butene-3-ol, 2-methyl-2-propene-1-ol, 4-pentene-1-ol, 1-pentene-3-ol, 2-pentene-1-ol, 2-methyl-3-butene-2-ol, 3-methyl-2-butene-1-ol, 3-methyl-3-butene-1-ol, 2-hexene-1-ol, 3-hexene-1-ol, 4-hexene-1-ol, 5-hexene-1-ol, 1-hexene-3-ol, 6-heptene-1-ol, 2-heptene-1-ol, 4-heptene-1-ol, 7-octene-1-ol, 2-octene-1-ol, 3-octene-1-ol, 5-octene-1-ol, 3-octene-2-ol and/or 1-octene-3-ol.    USE - Coolant composition is used for cooling fuel cell unit (both claimed).    ADVANTAGE - The coolant composition has excellent antifreeze property, and maintains the electrical conductivity of coolant for long period of time.    DETAILED DESCRIPTION - A coolant composition comprises fuel cell unit coolant containing aliphatic alcohol having unsaturated bond. The aliphatic alcohol is chosen from allyl alcohol, 2-butene-1-ol, 3-butene-1-ol, 1-butene-3-ol, 2-methyl-2-propene-1-ol, 4-pentene-1-ol, 1-pentene-3-ol, 2-pentene-1-ol, 2-methyl-3-butene-2-ol, 3-methyl-2-butene-1-ol, 3-methyl-3-butene-1-ol, 2-hexene-1-ol, 3-hexene-1-ol, 4-hexene-1-ol, 5-hexene-1-ol, 1-hexene-3-ol, 6-heptene-1-ol, 2-heptene-1-ol, 4-heptene-1-ol, 7-octene-1-ol, 2-octene-1-ol, 3-octene-1-ol, 5-octene-1-ol, 3-octene-2-ol, 1-octene-3-ol, 2-nonene-1-ol, 3-nonene-1-ol, 6-nonene-1-ol, 8-nonene-1-ol, 1-nonene-3-ol, 2-decene-1-ol, 4-decene-1-ol, 9-decene-1-ol, 3,7-dimethyl-6-octene-3-ol, 2-undecene-1-ol, 10-undecence-1-ol, 2-dodecene-1-ol, 2-propyne-1-ol, 2-butyne-1-ol, 1-butyne-3-ol, 3-butyne-1-ol, 1-pentyne-3-ol, 2-pentyne-1-ol, 3-pentyne-1-ol, 4-pentyne-1-ol, 4-pentyne-2-ol, 3-methyl-1-butyne-3-ol, 1-hexyne-3-ol, 3-hexyne-1-ol, 5-hexyne-3-ol, 2-hexyne-1-ol, 5-hexyne-1-ol, 3-methyl-1-pentyne-3-ol, 2-cyclohexene-1-ol, 2,4-hexadiene-1-ol, 1-heptyne-3-ol, 2-heptyne-1-ol, 3-heptyne-1-ol, 4-heptyne-2-ol, 5-heptyne-3-ol, 5-methyl-1-hexyne-3-ol, 3,4-dimethyl-1-pentyne-3-ol, 3-ethyl-1-pentyne-3-ol, 3,5-dimethyl-1-hexyne-3-ol, 3-octyne-1-ol, 1-octyne-3-ol, 2,7-octadienol, 3,6-dimethyl-1-heptyne-3-ol, 3-ethyl-1-heptyne-3-ol, 3-nonyne-1-ol, 2,6-nonadiene-1-ol, 3,6-nonadiene-1-ol, 1-cyclohexyl-2-butene-1-ol, 2-decyne-1-ol, 3-decyne-1-ol, 2,4-decadiene-1-ol, 4-ethyl-1-octyne-3-ol, 3,7-dimethyl-2,6-octadiene-1-ol, 10-undecyne-1-ol, 2,4-undecadiene-1-ol, 2,4-dodecadiene-1-ol, 3-methyl-1-pentene-4-yn-3-ol, 1-ethynyl-1-cyclohexanol, 2-butene-1,4-diol, 2-butyne-1,4-diol, 3-butene-1,2-diol, 2-methylene-1,3-propanediol, 7-octene-1,2-diol, 3,6-dimethyl-4-octyne-3,6-diol, 3-pentene-2-ol, 4-pentene-2-ol, 2-methyl-3-butene-1-ol, 5-hexene-2-ol, 3-methyl-1-pentene-3-ol, 4-methyl-3-pentene-1-ol, 4-methyl-2-cyclohexene-1-ol, 5-decene-1-ol, 3,7-dimethyl-6-octene-3-ol, 1,4-pentadiene-3-ol, 1,5-hexadiene-3-ol, 1,6-heptadiene-4-ol, 2-methyl-3-hexyne-2-ol, 1-ethynyl-1-cyclopentanol, 10-undecyne-1-ol, 1,5-hexadiene-3,4-diol and/or 3,5-cyclohexadiene-1,2-diol.