A pilot-scale horizontal subsurface flow constructed wetland was installed at the farm Mezaciruli, Zalenieki County, Jelgava region, in the middle part of Latvia, in August 2014 to improve stormwater quality collected from the farmyard and demonstrate applicability of constructed wetland as a convenient treatment option for contaminated surface runoff. The examined water treatment system consists of a sedimentation pond as a pretreatment plant, a water pump, a water distribution well, and a horizontal subsurface flow constructed wetland with the surface area of 160 m2 . During the observation period of 27 months (2014 – 2016) water quality parameters such as total suspended solids (TSS), nitrate nitrogen (NO3 -N), ammonium nitrogen (NH4 -N), total nitrogen (TN), orthophosphate phosphorus (PO4 -P), and total phosphorus (TP) were monitored and nutrient removal efficiency of the system was examined. Water samples collected at the inlet and outlet were compared. Monitoring results obtained during this study show that on average concentrations of TN and TP were reduced by 22% and 80%, respectively. However, nitrate - nitrogen concentrations on average increased by 10%, as during the time period from June, 2015 to March, 2016 reduction in NO3 -N concentrations was not observed.