A　free　cooling　unit　with　a　pump-driven　loop　heat　pipe　was　designed　for　internet　data　center　(IDC),　and　itscomponents　and　working　processes　were　described.　The　experiment　system　for　the　developed　prototype　was　built　and　thecomprehensive　studies　were　conducted.　The　experimental　results　demonstrate　that　the　coefficient　of　performance　(COP)　of　the　unit　can　reach　to　5.88　when　the　temperature　difference　between　IDC　indoor　and　outdoor　is　10°C,　and　it　will　be　10.41　when　the　temperature　difference　is　18°C.　When　mass　flow　rate　of　the　working　fluid　in　the　unit　changes　within　a　certain　range,　namely　the　gasification　rate　of　the　fluid　being　between　2%　and　50%,　there　is　no　significant　changes　for　the　heat　transfer　capacity.　And　there　is　an　approximately　linear　relationship　between　the　capacity　and　the　temperature　difference.　Meanwhile,　with　the　increase　of　the　system　resistance,　both　temperature　difference　of　the　fluid　between　inlet　and　outlet　of　the　evaporator　and　the　proportion　of　the　sensible　heat　to　the　capacity　also　increase.