In order to provide fundamental data on utilization of dehumidifier in greenhouses, a condensing type dehumidifier using ground water as a coolant was developed and tested dehumidification performance. The developed dehumidifier was applied to greenhouse with fog cooling system and effect of dehumidification on improvement of evaporative cooling efficiency was analyzed. Results of the dehumidifier performance test showed that dehumidification using ground water as a coolant was sufficiently possible in fog cooling greenhouse. When the set point temperature of greenhouse cooling was $32^{circ}C$ and as temperatures of ground water rose from $15^{circ}C;to;18^{circ}C,;21^{circ}C;and;24^{circ}C$, dehumidification rates decreased by $17.7\%,;35.4\%;and;52.8\%$, respectively. As flow rates of ground water reduced to $75\%;and;50\%$, dehumidification rates decreased by $12.1\%;and;30.5\%$, respectively. Cooling efficiency of greenhouse equipped with fog system was distinctly improved by artificial dehumidification. When the ventilation rate was 0.7 air exchanges per minute, dehumidification rates of the fog cooling greenhouse caused by natural ventilation were $53.9\%-74.4\%$ and they rose up to 75.4%-95.9% by operating the dehumidifier. In case of using the ground water of $18^{circ}C$ and flow rate of design condition, it was analyzed that whole fog spraying water can be dehumidified even if the ventilation rate is 0.36 exchanges per minute. As a utilization of dehumidifier, it is possible to improve cooling efficiency of fog system in naturally ventilated greenhouses.