The objective of this study was to investigate the effect of a fogging system on the microclimate of a rose greenhouse. The experiments were carried out in a multi-span plastic greenhouse, 106 wide by 205 m long, made of 11 spans. The fogging system consisted of a water softener and filters to prevent nozzle clogging, a water reservoir, pumps and a pressure regulator, and fog generating nozzles. Three nozzle lines with 82 fog generating nozzles were installed in each span of the plastic greenhouse. At each nozzle line, 82 fog generating nozzles were uniformly located at 2.5 m nozzle spacing. The fog generating nozzle parameters were determined to characterise the efficiency of the fogging system based on air flow rate and evaporation flow rate. The results showed that the fogging system was able to keep the air temperature inside the plastic greenhouse 6.6 degrees C lower than that outside. The average ventilation rate of the plastic greenhouse was 13.6 m(3)/s during the experimental period. The efficiency of the fogging system ranged from 11.7 to 80%. The efficiency of the fogging system increased as the difference between the dry-bulb temperature and wet-bulb temperature rose. The results indicated that air relative humidity inside the plastic greenhouse was increased by 25% on average by means of the fogging system examined in this study. The evaporation flow rate varied between 130 and 1223 g/h.m(2), whereas the air flow rate ranged from 39.3 to 298 kg/h.m(2). Fogging system efficiency increased linearly with evaporation flow rate and the absolute humidity difference between the inside and outside air.